Weird how I can't join pixelfed.social. I tried looking up how to register and the registration link is supposed to be there on the homepage according to their help center, but all I see is a login form
They might be at capacity and closed new signups for the moment. Being a federated network you can find a different instance, or set up your own if you want.
This being a federated network you should be able to follow and interact with users in other instances. That's what ActivityPub is about. I haven't checked Pixelfed specifically but it's how Mastodon works.
