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

The main instance is invite only for the time being but there are other open instances. Checkout full list here: https://fediverse.network/pixelfed?count=users

The most popular english open one seems to be: https://pixfed.com right now

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.

There is a Join link at https://pixelfed.org/ by the way.

How is my experience affected by being on a different instance to others?

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.