I'm already in an large co-writing project, actually. :/ Plus a million other things (including my own stories, which would be the case for many many people, methinks).
I'd note that if you do want to make a giant co-writing fic it'd take a great amount of time, effort and patience. And that'd go for everyone in it as well (who also happen to have the time and interest for it as well). Also... 'inviting only the authors I think would be good'... well, that's down to opinion, and sounds a bit iffy to me. :/
There is also a Writers Group around already if you look as well.
aw...thats to bad...anyway no problem and I'm thinking of making an authors club, inviting only the authors I think would be good for the job and then all together us make one big fanfic and then we could post it in the Pokemon or Other RP section and have others read it...It will be an invite only club...if I do that would you be interested?