Group Rooms are guest rooms that group creators can select to be the official group room. It costs 10c to create, and you must be a member of Habbo VIP to do so.
Members of your group automatically see your selected home room in their "me" tab on the navigator, and your room gets a 3 man sign next to its name. Group rooms are the only way for other people to drop furni in your room, it is also useful in that you can drop non-tradable furni such as wired or a few advertising furni, if they leave the group all their furni will return, however you can remove their admin even after they've dropped furni.
If you are the owner you can still use :pickall to pick only your furni up, however you can also use the command :ejectall to remove everyone's furni. If your room is locked, it will still be accessible to group members.