Referrals

Referrals is a simple system to reward players who refer others to your server. You can give rewards to both the referrer and the player who was referred. Rewards are command-based, so you can set commands to be run when someone refers someone else.
Upon joining the server, a player can type /referredby <player> to say who they were referred by. The commands for the specified reward are then run.
You can also set special rewards to be run when someone has referred a specific number of players. So for example, if player X refers someone for the first time, you can set an alternate set of commands to run to give them more rewards then they would normally get.

This mod can be run on the server side without clients needing to have it installed, so server owners can use it without making their players install it too.