They have a groups implementation on the road map. How long it takes, I don’t know … but mastodon aren’t fast generally, so it’s not happening soon in all likelihood.
Lemmy devs have taken a look at their spec for it and stated that it’s not at all consistent with lemmy’s implementation of communities over AP … so even when it lands it probably won’t help that much, at least at first.
Lemmy’s communities aren’t the only groups on the fediverse. The friendica/hubzilla groups have the same problem with mastodon’s handling of groups (they’re the only platforms apart from k/mbin that does both groups and users). Mike Macgirvin, the original dev of friendica/hubzilla, doesn’t have a high opinion of mastodon’s commitment to sensible standards (actually he’s been quite scathing in the past). I haven’t seen him comment on mastodon’s plans for groups, but given his prior criticism I take it as more likely than not that mastodon will do groups in their own way and in a way that won’t work well with the other group-based platforms.
Sorry for no links … this is all impressions and recollections.