Hello. A sign up, or SU, is an application to join an RP, and it's usually done by finding the OOC (Out of Character) thread with the same name as the RP. There, there should be an application form, or SU sheet, that you copy and fill in with your details and then post in the OOC thread. It should be as long and detailed as you can possibly make it, since that will raise your chances of getting a spot in the RP.
As for Unova Legends, I think the spots are already filled at the moment.