A Mastodon user can subscribe to a Lemmy Community in the same way they can Follow a Mastodon user e.g.

Mastodon user: @username@mastodoninstance.org Lemmy Community: @communityname@lemmyinstance.com

So they could now post to that Community and see every direct reply to that post and reply to those replies but they can’t currently just stumble across Lemmy Communities in their federated timeline because Lemmy can’t do that yet.