You don't sign up for the server. That link has a long list of sports accounts. Just find one you like, click follow, and it'll ask you to enter your Mastodon username. Then you can follow.
Since Twitter officially communicated (see also MacRumors, MacStories, The Verge) to ban similar third party Twitter clients to use their API it might be interesting to read a bit more on the options out there such as Mastodon and all the third party apps that are available in the App Store or in TestFlight. See also my Mastodon post.