If you look at the website and find one of the communities you can subscribe to it. To find get to that community do: lemmy.ml/c/[the community]@[the website URL]. For example: https://lemmy.ml/c/science@mander.xyz

Alternatively, I just figured out an easier way. Click the Join Lemmy button at the bottom of the page while on your signed in account. You can then go on the different servers while logged into your main account

I don’t think migration is possible, you would just have to make an alt account. Although mastodon does have this feature, so it is possible that it gets added one day