Implemented basic structure

lib/data/dto/group.dart

@@ -0,0 +1,9 @@
+import 'package:game_tracker/data/methods/dto/player.dart';
+
+class Group {
+  final String id;
+  final String name;
+  final List<Player> members;
+
+  Group({required this.id, required this.name, required this.members});
+}