diff --git a/src/main/java/mops/gruppen2/controller/Gruppen2Controller.java b/src/main/java/mops/gruppen2/controller/Gruppen2Controller.java
index 50be0f2..307f5ae 100644
--- a/src/main/java/mops/gruppen2/controller/Gruppen2Controller.java
+++ b/src/main/java/mops/gruppen2/controller/Gruppen2Controller.java
@@ -117,6 +117,15 @@ public class Gruppen2Controller {
         throw new ResponseStatusException(HttpStatus.NOT_FOUND, "Group not found");
     }
 
+    @RolesAllowed({"ROLE_orga", "ROLE_studentin", "ROLE_actuator)"})
+    @PostMapping("/detailsBeitreten")
+    public String joinGroup(KeycloakAuthenticationToken token, Model model, @RequestParam(value = "id") Long id) throws EventException {
+        model.addAttribute("account", keyCloakService.createAccountFromPrincipal(token));
+        Account account = keyCloakService.createAccountFromPrincipal (token);
+        controllerService.addUser(account,id);
+        return "redirect:/gruppen2/";
+    }
+
     @RolesAllowed({"ROLE_orga", "ROLE_studentin", "ROLE_actuator)"})
     @GetMapping("/detailsSearch")
     public String showGroupDetailsNoMember (KeycloakAuthenticationToken token, Model model, @RequestParam (value="id") Long id) throws EventException {
diff --git a/src/main/java/mops/gruppen2/domain/Exceptions/GroupDoesNotExistException.java b/src/main/java/mops/gruppen2/domain/Exceptions/GroupDoesNotExistException.java
deleted file mode 100644
index 785b901..0000000
--- a/src/main/java/mops/gruppen2/domain/Exceptions/GroupDoesNotExistException.java
+++ /dev/null
@@ -1,9 +0,0 @@
-package mops.gruppen2.domain.Exceptions;
-
-import mops.gruppen2.domain.event.Event;
-
-public class GroupDoesNotExistException extends EventException {
-    public GroupDoesNotExistException(String msg) {
-        super(msg);
-    }
-}
diff --git a/src/main/java/mops/gruppen2/domain/event/AddUserEvent.java b/src/main/java/mops/gruppen2/domain/event/AddUserEvent.java
index 2d9d470..a97845c 100644
--- a/src/main/java/mops/gruppen2/domain/event/AddUserEvent.java
+++ b/src/main/java/mops/gruppen2/domain/event/AddUserEvent.java
@@ -3,7 +3,10 @@ package mops.gruppen2.domain.event;
 import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
+import mops.gruppen2.domain.Exceptions.EventException;
+import mops.gruppen2.domain.Exceptions.UserAlreadyExistsException;
 import mops.gruppen2.domain.Group;
+import mops.gruppen2.domain.Role;
 import mops.gruppen2.domain.User;
 
 /**
@@ -24,8 +27,14 @@ public class AddUserEvent extends Event {
         this.email = email;
     }
 
-    public void apply(Group group) {
+    public void apply(Group group) throws EventException{
         User user = new User(this.user_id, this.givenname, this.familyname, this.email);
+
+        if (group.getMembers().contains(user)){
+            throw new UserAlreadyExistsException("Der User existiert bereits");
+        }
+
         group.getMembers().add(user);
+        group.getRoles().put(user_id, Role.MEMBER);
     }
 }
diff --git a/src/main/java/mops/gruppen2/domain/event/DeleteUserEvent.java b/src/main/java/mops/gruppen2/domain/event/DeleteUserEvent.java
index d7cff68..1afab84 100644
--- a/src/main/java/mops/gruppen2/domain/event/DeleteUserEvent.java
+++ b/src/main/java/mops/gruppen2/domain/event/DeleteUserEvent.java
@@ -1,6 +1,8 @@
 package mops.gruppen2.domain.event;
 
 import lombok.*;
+import mops.gruppen2.domain.Exceptions.EventException;
+import mops.gruppen2.domain.Exceptions.UserNotFoundException;
 import mops.gruppen2.domain.Group;
 import mops.gruppen2.domain.User;
 import mops.gruppen2.domain.Group;
@@ -15,7 +17,7 @@ public class DeleteUserEvent extends Event {
         super(group_id, user_id);
     }
 
-    public void apply(Group group) {
+    public void apply(Group group) throws EventException {
         for (User user : group.getMembers()) {
             if (user.getUser_id().equals(this.user_id)) {
                 group.getMembers().remove(user);
@@ -23,5 +25,6 @@ public class DeleteUserEvent extends Event {
                 return;
             }
         }
+        throw new UserNotFoundException("Der User existiert nicht");
     }
 }
diff --git a/src/main/java/mops/gruppen2/domain/event/Event.java b/src/main/java/mops/gruppen2/domain/event/Event.java
index 6e9406a..17a00f9 100644
--- a/src/main/java/mops/gruppen2/domain/event/Event.java
+++ b/src/main/java/mops/gruppen2/domain/event/Event.java
@@ -6,6 +6,7 @@ import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 import lombok.Setter;
+import mops.gruppen2.domain.Exceptions.EventException;
 import mops.gruppen2.domain.Group;
 
 
@@ -31,5 +32,6 @@ public class Event {
     String user_id;
 
 
-    public void apply(Group group){}
+    public void apply(Group group) throws EventException {
+    }
 }
diff --git a/src/main/java/mops/gruppen2/domain/event/UpdateRoleEvent.java b/src/main/java/mops/gruppen2/domain/event/UpdateRoleEvent.java
index f115c97..eeea3d9 100644
--- a/src/main/java/mops/gruppen2/domain/event/UpdateRoleEvent.java
+++ b/src/main/java/mops/gruppen2/domain/event/UpdateRoleEvent.java
@@ -24,7 +24,10 @@ public class UpdateRoleEvent extends Event {
         this.newRole = newRole;
     }
 
-    public void apply(Group group) {
+    public void apply(Group group) throws UserNotFoundException{
+        if (!group.getRoles().containsKey(user_id)){
+            throw new UserNotFoundException("Der User wurde nicht gefunden");
+        }
         group.getRoles().put(this.user_id, this.newRole);
     }
 
diff --git a/src/main/resources/templates/detailsNoMember.html b/src/main/resources/templates/detailsNoMember.html
index 37a3c4a..44f1dfd 100644
--- a/src/main/resources/templates/detailsNoMember.html
+++ b/src/main/resources/templates/detailsNoMember.html
@@ -29,7 +29,6 @@
         <div class="row">
             <div class="col-9">
                 <div class="shadow-sm p-4" style="border: 10px solid aliceblue; background: aliceblue">
-                    <form action="/" method="get">
                         <h1 style="color: dodgerblue; font-weight: bold" th:text="${group.getTitle()}"></h1>
                         <p style="font-weight: bold">
                             <span class="badge badge-pill badge-dark" style="background: darkslategray" th:if="${group.getVisibility() == group.getVisibility().PRIVATE }">Private Gruppe</span>
@@ -39,10 +38,11 @@
                         <p th:text="${group.getDescription()}"></p>
                         <div class="form-group">
                             <div class="text-right">
-                                <button class="btn btn-primary" style="border-style: none;">Gruppe beitreten</button>
+                                <form method="post" action="/gruppen2/detailsBeitreten">
+                                    <button type="submit" th:href="@{/gruppen2/detailsBeitreten(id=${group.getId()})}" th:name="id" th:value="${group.id}" class="btn btn-primary" style="border-style: none;">Gruppe beitreten</button>
+                                </form>
                             </div>
                         </div>
-                    </form>
                 </div>
             </div>
         </div>
diff --git a/src/test/java/mops/gruppen2/domain/event/AddUserEventTest.java b/src/test/java/mops/gruppen2/domain/event/AddUserEventTest.java
new file mode 100644
index 0000000..649166f
--- /dev/null
+++ b/src/test/java/mops/gruppen2/domain/event/AddUserEventTest.java
@@ -0,0 +1,35 @@
+package mops.gruppen2.domain.event;
+
+import mops.gruppen2.domain.Exceptions.EventException;
+import mops.gruppen2.domain.Exceptions.UserAlreadyExistsException;
+import mops.gruppen2.domain.Group;
+import mops.gruppen2.domain.User;
+import org.junit.jupiter.api.Test;
+
+import static mops.gruppen2.domain.Role.MEMBER;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.*;
+
+class AddUserEventTest {
+
+    @Test
+    public void userAllreadyExistExeption() throws EventException {
+        Group group = new Group();
+
+        User user = new User("user1","Stein", "Speck", "@sdasd");
+
+        group.getMembers().add(user);
+
+        Event event1 = new AddUserEvent(4L, "user2", "Rock", "Roll", "and");
+        event1.apply(group);
+
+        Event event2 = new AddUserEvent(4L, "user1", "Rock", "Roll", "and");
+
+        assertThrows(UserAlreadyExistsException.class, ()->
+                event2.apply(group)
+                );
+        assertThat(group.getMembers().size()).isEqualTo(2);
+    }
+
+
+}
\ No newline at end of file
diff --git a/src/test/java/mops/gruppen2/domain/event/DeleteUserEventTest.java b/src/test/java/mops/gruppen2/domain/event/DeleteUserEventTest.java
index 59fc2de..6f3c67a 100644
--- a/src/test/java/mops/gruppen2/domain/event/DeleteUserEventTest.java
+++ b/src/test/java/mops/gruppen2/domain/event/DeleteUserEventTest.java
@@ -1,5 +1,8 @@
 package mops.gruppen2.domain.event;
 
+import mops.gruppen2.domain.Exceptions.EventException;
+import mops.gruppen2.domain.Exceptions.UserAlreadyExistsException;
+import mops.gruppen2.domain.Exceptions.UserNotFoundException;
 import mops.gruppen2.domain.Group;
 import mops.gruppen2.domain.User;
 import org.junit.jupiter.api.Test;
@@ -11,7 +14,7 @@ import static org.junit.jupiter.api.Assertions.*;
 class DeleteUserEventTest {
 
     @Test
-    void apply() {
+    void applyDeleteUser() throws EventException {
         Group group = new Group();
 
         User user = new User("user1","Stein", "Speck", "@sdasd");
@@ -32,4 +35,22 @@ class DeleteUserEventTest {
         assertThat(group.getRoles().size()).isEqualTo(1);
 
     }
+
+    @Test
+    void userDoesNotExistExeption() {
+        Group group = new Group();
+
+        User user = new User("user1","Stein", "Speck", "@sdasd");
+
+        group.getMembers().add(user);
+        group.getRoles().put("user1", MEMBER);
+
+        Event event = new DeleteUserEvent(17L,"user5");
+
+        assertThrows(UserNotFoundException.class, ()->
+                event.apply(group)
+        );
+
+        assertThat(group.getMembers().size()).isEqualTo(1);
+    }
 }
\ No newline at end of file
diff --git a/src/test/java/mops/gruppen2/service/GroupServiceTest.java b/src/test/java/mops/gruppen2/service/GroupServiceTest.java
index 03fd9a1..c67a227 100644
--- a/src/test/java/mops/gruppen2/service/GroupServiceTest.java
+++ b/src/test/java/mops/gruppen2/service/GroupServiceTest.java
@@ -1,6 +1,5 @@
 package mops.gruppen2.service;
 
-import mops.gruppen2.domain.Exceptions.GroupDoesNotExistException;
 import mops.gruppen2.domain.Group;
 import mops.gruppen2.domain.GroupType;
 import mops.gruppen2.domain.Visibility;
@@ -29,36 +28,6 @@ class GroupServiceTest {
         groupService = new GroupService(mock(EventService.class), eventRepository);
     }
 
-    @Disabled
-    @Test
-    void applyEventOnGroupThatIsDeleted() throws Exception {
-        List<Event> eventList = new ArrayList<>();
-
-        eventList.add(new CreateGroupEvent(1L,"Ulli", null, GroupType.LECTURE, Visibility.PRIVATE));
-
-        eventList.add(new DeleteGroupEvent(10, "loescher78"));
-
-        eventList.add(new AddUserEvent(10L, "Ulli", "Ulli", "Honnis", "FC@B.de"));
-
-
-        Assertions.assertThrows(GroupDoesNotExistException.class, () -> {
-            groupService.projectEventList(eventList);
-        });
-    }
-
-    @Disabled
-    @Test
-    void returnDeletedGroup() throws Exception {
-        List<Event> eventList = new ArrayList<>();
-
-        eventList.add(new CreateGroupEvent(1L, "Prof", null, GroupType.LECTURE, Visibility.PRIVATE));
-
-        eventList.add(new DeleteGroupEvent(1L, "loescher78"));
-
-        List<Group> list = new ArrayList<>();
-
-        assertThat(groupService.projectEventList(eventList)).isEqualTo(list);
-    }
 
     @Test
     void rightClassForSucsessfulGroup() throws Exception {