Like others have said, you can follow accounts from those servers. If they present themselves as group actors, as Lemmy (unlike Mbin for example) doesn't allow following non-group actors.

You can't however just go to their website and log in with your Lemmy account. ActivityPub doesn't have a built-in mechanism for this, but some platforms like Mastodon iirc have a solution and there have been efforts made to standardize something. But there's nothing supported by the Lemmy side of the fediverse yet afaik.

If you want to subscribe to a group actor from Lemmy but can't find them, try searching for the full URL from its home instance. That should tell it to go and fetch the actor. If you want a Lemmy-like experinece but also the ability to follow non-group actors, switch to Mbin. Same applies there, if you can't find an actor on your instance, search for their full URL on the search page and it should fetch them from their home server.