Don't worry that they're asking for a commitment from you, because they're not. Right now they want your ideas, your requests, your prompts (M/M only). (The commitment bit comes later when they open the lists so you can all go claim a story or three to write. ;))
You can post up to three prompts, though they must be for three different pairings/characters. They can all be from the same fandom (ie, you may request Spike/Xander, Spike/Angelus and Xander/Giles (or even Spike/Xander, Spike/Xander/Oz and Spike/Xander/Angel) in the Buffy Fandom but you may not make 3 Spike/Xander requests).