With the rewards rank you can easily add a role which will be added to members when they reach a certain amount of invites!
To add a new rank, this is really easy, simply type
+addRank <amount> <role> [explaination]. Amount is the amount of messages that the member need to obtain this role and @role is the role which will be assigned to the member when he reach the goal.
To remove a reward rank, you simply need to type
+removeRank @role. @role is the role that you want to remove.
To list all reward ranks you need to type
+ranks, it will display a list with all reward ranks and the number of messages required for each one.