add functionality for group_ids to User class

Co-Authored-By: tomvahl <tomvahl@users.noreply.github.com>

src/main/java/mops/gruppen2/service/TeilnehmerService.java

@@ -0,0 +1,26 @@
+package mops.gruppen2.service;
+
+import mops.gruppen2.domain.User;
+import mops.gruppen2.domain.event.Event;
+import org.springframework.stereotype.Service;
+
+import java.util.List;
+
+@Service
+public class TeilnehmerService {
+    GroupService groupService;
+    EventService eventService;
+
+    public TeilnehmerService(GroupService groupService, EventService eventService){
+        this.eventService = eventService;
+        this.groupService = groupService;
+    }
+
+    public void assignGroups(User user){
+        List<Event> events = eventService.findAllEvents();
+
+        for (Event event: events) {
+            if(user.getUser_id().equals(event.getUser_id())) user.addGroup(event.getGroup_id());
+        }
+    }
+}