thegreenguy

Just found out Matrix can do this, although you need to host your own server. There is also Beeper for a fully-managed product, but you need to wait to get access.

I would also add that Lemmy is part of the fediverse, meaning it is federated. Federation means all instances "talk" to all instances (unless they defederate), so you aren't limited only to the content on one instance (or in some cases not even Lemmy, case in point: I'm posting this from my kbin.social account).