There was a thread like this on UMK.com. Basically, you just list names of people who you have good games with. The idea is to list people most players don't know about, but you can list anyone you think is a good enough player that he will challenge others. The purpose of the thread is to get this game more active by making it known there's decent players you can play other than the known players.
All on 2006 XBLA UMK3:
-dauggette: Has some lag, but decent player.
-oO 73r3aK3r Oo: Pretty good, played him in ranked a bunch.
-The V i P 23 HD: Probably the best Scorpion player I've played online.
-warrior235: Great player, never even heard of him until the other day.
-drikxz11: Had good matches with this guy. Slight lag though.
-BabyJesusALITY: He seems to be on a lot, and he's decent.
-XxItSickxX: He's aight.
All on 2006 XBLA UMK3:
-dauggette: Has some lag, but decent player.
-oO 73r3aK3r Oo: Pretty good, played him in ranked a bunch.
-The V i P 23 HD: Probably the best Scorpion player I've played online.
-warrior235: Great player, never even heard of him until the other day.
-drikxz11: Had good matches with this guy. Slight lag though.
-BabyJesusALITY: He seems to be on a lot, and he's decent.
-XxItSickxX: He's aight.