I don’t think you can follow individual account with lemmy account? But if you can follow individual user on lemmy, then you should be able to follow people on mastodon.

I am not sure you can from Lemmy, but you can from kbin. You would have to search @username@mastodon.social, and the user should come up.

I don’t know if this capability has been added to the Artemis app yet for kbin - it’s still in beta.