uscf

Well, the ones I've been to if you show up during registration the day of the tournament and you're not a member, they just automatically make you a club member
Sometimes that includes a small fee, sometimes not. They should say so in the advertisement of the tournament (online or in Chess Life).
I just assumed you tried to register online and it didn't work or something.

You have to be a member of the US Chess group on the site. To do that you have to fill out the authentication form first and wait until the USCF verifies your information before you can take part in any events.
https://www.chess.com/news/view/uscf-member-authentication-9266