Dividing up the work would likely be best, and having someone who has the say over who gets what, so that people don't argue over saying "I wanted that part." Google Docs would make a good spot for everyone to put their pages, and then turning those over to the person in charge to make one large compendium. From there, the people who actually made the pages could be in charge of upkeep (things like Primals and Ascians would require upkeep, others like some NPCs, probably would need considerably less upkeep), or a group could be put in charge of upkeep.