While both Lemmy and Mastodon use ActivityPub, the way they send ActivityPub messages to each other differs quite a bit. There is not a simple or quick solution to this. However recently I read about Lemmy having improved integration with https://a.gup.pe/, a service that adds Groups support to Mastodon so that might be something you want to try out.
Mastodon having so many users makes it a tempting target for integration but federation with software that uses Groups properly (e.g. PeerTube, Friendica) is likely to be easier to achieve.
While both Lemmy and Mastodon use ActivityPub, the way they send ActivityPub messages to each other differs quite a bit. There is not a simple or quick solution to this. However recently I read about Lemmy having improved integration with https://a.gup.pe/, a service that adds Groups support to Mastodon so that might be something you want to try out.
Mastodon having so many users makes it a tempting target for integration but federation with software that uses Groups properly (e.g. PeerTube, Friendica) is likely to be easier to achieve.