All "Sytes" Servers have a special reserved slot for VIP,Admin Members; however, once all server slots are occupied ( 30/30 ), a usual connection method does not work anymore. In that case, you will need to connect through the game console. Open the game console ( press the ~ button ) and type in: connect SERVERIP:PORT and press the Enter button. If it still gives you Server is full. open the game console and type in retry and press ENTER. That happens because somebody tries to connect at the same time as you, and it takes some time for the server to free up a slot for you.
Once you successfully completed your payment, it takes some time for the system to process your details (not longer than 60 minutes; in most cases, the activation is instant) before your account becomes active.
It means you are using a password protected Nick, i.e., the Nick is registered. If you are the owner of that Nick or/and you know the right Nick password, simply type the password in your game console ( press the ~ button and write ) setinfo _pw PASSWORD (note that there is a space between setinfo and _pw, replace the word PASSWORD with a proper Nick password).