The idea is, you can put guests of a room into sub-groups. When added to a sub group, those guests will only be able to see and hear others in that same sub group.
Guests can be assigned to multiple subgroups. Groups can be specified via the URL using &group=1,5,6 or/and the director can dynamically assign sub-groups, as seen in the below image.
If not in a group, that guest will still see/hear everyone, regardless of which group they are in, even if a guest in another group may not be able to see/hear that guest back.
Scenes can be put into groups as well, via the URL group option, such &group=3, but the director will not be able to dynamically change which group a scene is in. Not yet at least.
Using this group function is an alternative to transfer rooms, however it's perhaps less secure, as a guest could just tinker with their URL parameters or just refresh their page to perhaps see everyone in the room again.
&groupaudio can be used to enable audio in-between different groups, instead of audio being group-specific. Useful for blind-dating show formats or such.