I'm not trying to take anything away from xboxachievements.org but I highly recommend to anyone looking for multiplayer achievements to head over to trueachievements.com since they have scheduled boosting sessions for specific achievements.
I am not trying to take away anything from this website, I believe xbox360achievements.org has a wealth of information when you're looking for walkthroughs or information on specific achievements.
However, trueachievements.com has specific gaming sessions depending on the game or achievement you are looking for. Instead of random people hopping in here and saying "Hey add me" and then never return you can schedule a gaming session when its a good time for you.
You can also browse the Saints Row online gaming sessions and you can join the one that matches up with you schedule.
First boosting session I joined last night I dropped off 150 big ass chains in a ranked match. Everyone was experienced with game modes. number of people needed to start a ranked match and had no problem helping each other out.