They can but it’s a bit wonky. You can “subscribe” to lemmy communities by following the community as if it were a username, so for example you’d follow @chat@beehaw.org to follow !chat@beehaw.org. However, the result is that every single post and comment made in chat gets boosted (“retweeted”) into your feed, which is… not ideal.

You can also respond to any lemmy post by pulling it up in mastadon and replying to it with your mastadon account. You can just search the link to a lemmy thread or post in the mastadon search bar to bring it up and reply from there.