That is open to interpretation. What the spec says:
Subscribers can subscribe to a feed with multiple authors, such as a group, a search feed,
or a tag feed.
We have tags, of course. The way StatusNet implemented groups is an expression of this functionality, but it comes with technical debt, so Mastodon may want a different way to express it.
I personally do not miss group functionality, so I would prioritize other features, especially to let the fediverse develop stronger opinions about groups, and how they will interact with other OStatus projects.