Over the years there's been a ton of achievements that I've stumbled across that require some multiplayer work, but I've had no friends who are dedicated enough achievement-wise to want to help me out and get them. It'd be nice if, on the page for an achievement (like this one:
The Still Alive achievement in Portal 2 worth 33 pointsComplete Course 4 with neither you nor your co-op partner dying ), there was a button to click to add it to a boost list so that anyone else who wants to do it could shoot me a message and we could work on it together.
An easy way to do it would be to have a list on every achievement page with people that have it in their to-do list -- it reuses existing site tech and is a straightforward thing to wrap your head around, as it's easy to assume that someone who has a multiplayer achievement on their to-do list wants people to do it with!