It normally is an automatism. You have to join if there are less than 15 People on the Server and stay then until there are more than 45 Players on the Server. The Server then sends a message to you like “Thank you for helping to populate the Server. Your Server reputation is now….” But not only one time you have to do it on 5 of 7 Days every week 😀 If someone ask for it and he thinks he deserved a VIP Slot for helping to populate the Server he can add me or you can contact me then. I will take a look and add the VIP Slot then manually.
You also can see it in the Admin Dashboard if you look at the player. There you see in the History “Player Successfully Populated Server”…
Then there are also some benefits for Players with a good reputation. You can find all about it here: http://jah-warriors.com/servers/reputation-faq/