In actuality, I'm not even sure how you would use it in the way that it looks in the one example in #ActivityStreams, where it exists without any sort of attached unique identifier.
The example is descriptive, but not usable as written to infer behavior or meaning. I don't even have a good way to talk about who might be in said group: which actors or objects might be members?
I could use a collection, maybe, but what if I want to scope it to members of a server?