The uRPS server uses a whitelist. Only players on this list have permission to build and destroy on the server (as opposed to a blacklist, which lists people who are banned from it). Getting on the whitelist is not usually hard, as long as you are not a dick with intentions of griefing.
Communicating with Admins to get on to the whitelist should be done in the server's forum thread. Priority is given to active RPS members and those recommended by current players. When asking to be whitelisted in the server forum thread make sure you tell us your Minecraft name, and confirm that you have read the FAQ.
While you are waiting to be whitelisted, feel free to join the server and take a look around. Introducing yourself on the server and being friendly will certainly help you be welcomed onto it.
Note that once accepted on the whitelist, you will normally receive Probation status. This is essentially identical to Builder, but it gives us more tools to track you until we are confidant we can trust you. There is no difference between the commands and abilities of Probation versus Builder.