My memory isn't good enough to remember how to bootstrap a game. I believe TetriNET didn't have a forum to play against strangers, but I could be wrong. I played only with friends who would initiate the game with an instant message group chat first.

Did your league organize on IRC to bootstrap games with the host's IP address? Is there a webpage with league standings still around?

Here you go :) http://www.oocities.org/timessquare/7785/tetrinet/ladder.htm