(I'm back)
The !fediverse group is for all things related to the whole Fediverse, !gnusocial is about the gnusocial part of the fediverse and in !fedgroups it's all about groups. If you create a new group or if a group moves to another instance or you want people to know about a group you post to !fedgroups.

Some Instances have a group for their instance, too.