Hello there,
It's been a while since I shared something so I share with you something I found on my hard drive which is coded by me.
It is an invite system. The player's of your server can login and take a special link. After that they can share this link with their friends, and then for each unique visit they earn prizes.
You need two databases. The server's db and the database that will save the ips who are invited.
In invite.php, you can write the quantity and the item id of the reward.
It is coded in PHP and I hope it works. If something is wrong reply here and I will do my best to fix it.
Download
Have Fun.