MyBB supports referral links without any modifications, and there's a "show referred users" tool in the Admin CP, but you'll need a modification to show referrals in profiles (
this one for 1.2 could be updated pretty easily).
To make a referral link, add the "referrer" or "referrername" parameter to any board URL, like so:
mybb.example.com/index.php?referrer=12345
mybb.example.com/index.php?referrername=somebody
The referrer ID will then be stored in a cookie, and used to auto-fill the Referrer field when the new user registers.
Edit: The author of that plugin said he'll be updating it soon:
http://community.mybboard.net/post-24353...#pid243531