You could try www.thebackpacker.com. They have a "trips" section where people announce trips. I don't know if they let anyone go who wants to, but it might be worth checking out. I hadn't been to the site in years, but just checked it to make sure it was still there. I did once go on a trip that was scheduled there, but found out about it from another source. The forums there didn't appeal to me due to the trolls.