For those of you who administrate a 7th Column Group at Bungie.net, you'll notice a new set of links there now that allow you to link up to seven (appropriate number, no?) different clans to your Group. It can be a clan that includes your members, or an allied clan, or even a rival clan. Links to those clans' statistics pages will appear on your Group's pages.
In order to link a clan, you have to be a member of at least one clan (which you need to do in Halo 2, not on Bungie.net) and have your Gamertag linked to your Bungie.net login. Once you've done so, Halo 2 will display a Bungie.net logo behind your gamertag during matchmaking.
Thanks rapture for the heads-up on cookedgamers.