There is a reason AG as no section for Multiplayer: It basically has few games that are Multiplayer, too few to make a games section from it. I can only think of Colony and Platform Racing only. Maybe there are a few others, but AG users don't really have an interest for them. I think it's because to make a multiplayer game on another website takes more work, and cooperation. And some websites have a bigger interest in multiplayer, and therefore have more.
There is already a tag for it, there are just not enough games for it to appear on the front page yet. You can check out the multiplayer games here.
As for as an actual section for it, I doubt they will add any more sections. The thing is, most sport, multiplayer, etc. games, will fit under another genre just fine. So there isn't really a good reason for them to add more.
