I know this game is old, and most people may figure this out, but these other guides didn't mention something had trouble with.
When having someone look for a ranked match I created, my match never showed up in the match list or came up in quick match (even though I was the only one with a match going.) What I figured out was the minimum player count need to be at least 4 for the game to show up for other players in ranked matches. It's true you only need 2 players for Deathmatch, and you can start the game with only two, but you need to allow 4 or the match will now show up for your boosting partner. You do not need to worry about this in player matches of course. Just ranked.
As soon as I changed the minimum player count to 4, people could get into my match.