Presumably if the protocol becomes popular you will have public instances that anyone* can sign up to, the same way other federated services work now.

(It's approximately as hard for a layman user to sign up to a Mastodon instance as it is to sign up for X. The fact that most layman users don't do that is a separate issue.)

Yes, you can try it without running your own instance. You will need to use someone else's instance.