diff --git a/back/src/Model/Group.ts b/back/src/Model/Group.ts
index ed09b0cd..16cd5ffe 100644
--- a/back/src/Model/Group.ts
+++ b/back/src/Model/Group.ts
@@ -7,13 +7,13 @@ export class Group {
     static readonly MAX_PER_GROUP = 4;
 
     private id: string;
-    private users: UserInterface[];
+    private users: Set<UserInterface>;
     private connectCallback: ConnectCallback;
     private disconnectCallback: DisconnectCallback;
 
 
     constructor(users: UserInterface[], connectCallback: ConnectCallback, disconnectCallback: DisconnectCallback) {
-        this.users = [];
+        this.users = new Set<UserInterface>();
         this.connectCallback = connectCallback;
         this.disconnectCallback = disconnectCallback;
         this.id = uuid();
@@ -24,7 +24,7 @@ export class Group {
     }
 
     getUsers(): UserInterface[] {
-        return this.users;
+        return Array.from(this.users.values());
     }
 
     getId() : string{
@@ -42,8 +42,8 @@ export class Group {
             x += user.position.x;
             y += user.position.y;
         });
-        x /= this.users.length;
-        y /= this.users.length;
+        x /= this.users.size;
+        y /= this.users.size;
         return {
             x,
             y
@@ -51,45 +51,27 @@ export class Group {
     }
 
     isFull(): boolean {
-        return this.users.length >= Group.MAX_PER_GROUP;
+        return this.users.size >= Group.MAX_PER_GROUP;
     }
 
     isEmpty(): boolean {
-        return this.users.length <= 1;
+        return this.users.size <= 1;
     }
 
     join(user: UserInterface): void
     {
         // Broadcast on the right event
         this.connectCallback(user.id, this);
-        this.users.push(user);
+        this.users.add(user);
         user.group = this;
     }
 
-    isPartOfGroup(user: UserInterface): boolean
-    {
-        return this.users.includes(user);
-    }
-
-    /*removeFromGroup(users: UserInterface[]): void
-    {
-        for(let i = 0; i < users.length; i++){
-            let user = users[i];
-            const index = this.users.indexOf(user, 0);
-            if (index > -1) {
-                this.users.splice(index, 1);
-            }
-        }
-    }*/
-
     leave(user: UserInterface): void
     {
-        const index = this.users.indexOf(user, 0);
-        if (index === -1) {
-            throw new Error("Could not find user in the group");
+        let success = this.users.delete(user);
+        if (success === false) {
+            throw new Error("Could not find user "+user.id+" in the group "+this.id);
         }
-
-        this.users.splice(index, 1);
         user.group = undefined;
 
         // Broadcast on the right event
@@ -102,8 +84,8 @@ export class Group {
      */
     destroy(): void
     {
-        this.users.forEach((user: UserInterface) => {
+        for (let user of this.users) {
             this.leave(user);
-        })
+        }
     }
 }
diff --git a/back/src/Model/World.ts b/back/src/Model/World.ts
index 2cb92dd2..e4ce5c32 100644
--- a/back/src/Model/World.ts
+++ b/back/src/Model/World.ts
@@ -20,7 +20,7 @@ export class World {
 
     // Users, sorted by ID
     private readonly users: Map<string, UserInterface>;
-    private readonly groups: Group[];
+    private readonly groups: Map<string, Group>;
 
     private readonly connectCallback: ConnectCallback;
     private readonly disconnectCallback: DisconnectCallback;
@@ -35,7 +35,7 @@ export class World {
                 groupDeletedCallback: GroupDeletedCallback)
     {
         this.users = new Map<string, UserInterface>();
-        this.groups = [];
+        this.groups = new Map<string, Group>();
         this.connectCallback = connectCallback;
         this.disconnectCallback = disconnectCallback;
         this.minDistance = minDistance;
@@ -45,7 +45,7 @@ export class World {
     }
 
     public getGroups(): Group[] {
-        return this.groups;
+        return Array.from(this.groups.values());
     }
 
     public getUsers(): Map<string, UserInterface> {
@@ -99,7 +99,7 @@ export class World {
                         user,
                         closestUser
                     ], this.connectCallback, this.disconnectCallback);
-                    this.groups.push(group);
+                    this.groups.set(group.getId(), group);
                 }
             }
 
@@ -132,11 +132,10 @@ export class World {
         if (group.isEmpty()) {
             this.groupDeletedCallback(group.getId(), user);
             group.destroy();
-            const index = this.groups.indexOf(group, 0);
-            if (index === -1) {
-                throw new Error("Could not find group");
+            if (!this.groups.has(group.getId())) {
+                throw new Error("Could not find group "+group.getId()+" referenced by user "+user.id+" in World.");
             }
-            this.groups.splice(index, 1);
+            this.groups.delete(group.getId());
         } else {
             this.groupUpdatedCallback(group);
         }
diff --git a/back/tsconfig.json b/back/tsconfig.json
index 51858b84..397bb8a2 100644
--- a/back/tsconfig.json
+++ b/back/tsconfig.json
@@ -4,6 +4,7 @@
     /* Basic Options */
     // "incremental": true,                   /* Enable incremental compilation */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019', 'ES2020', or 'ESNEXT'. */
+    "downlevelIteration": true,
     "module": "commonjs",                     /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', 'es2020', or 'ESNext'. */
     // "lib": [],                             /* Specify library files to be included in the compilation. */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */