It's Spander, with Giles thrown in. ^_^ Giles let Spike out of the bathtub, on the condition that they enter into a sexual relationship - think dom/sub - and then Spike suggests they get Xander to join in with their fun, 'cause Spike can smell that Xander's interested. It's all hush-hush from the rest of the scoobies, and Giles is almost Ripper-ish in it, but it's v.good.
The community was some kind of request comm - I'm not sure if it was kink requests, or just fic requests in general, but I remember they had a list of ones which had been fulfilled. Something like, you have to fulfill someone else's request before being allowed to make a request of your own.
Does this ring any bells for anyone?
Anyway thanks to anyone who replies! And sorry again if this isn't allowed!
Edit: thank you devilishyaya, the fic is 'Sharing' by lilithbint (http://lilithbint.livejournal.com/53138.html#cutid1)