Second this--it's always more fun to write in a group, even if you all are sitting quietly grumbling at your various writing implements.

I've actually pseudo-volunteered (because I'm on the board of directors of the local friends of the library--completely accidentally) to run something like that at the local library this November. Given my proclivities, anyone who does show up is likely to finish, if only through exposure to my programmer-strength coffee.

One of the things which I do--you may find it helpful; you may not--is to 'handicap' myself with the story. I do this through setting a condition before I start writing that limits my scope to some extent--which in my case actually helps a lot, given my usual problem is having too many ideas in my head, which interrupts my flow.

For example, one year I set out to write a zombie novel, with the condition that the zombies be biologically plausible (cleared that with a microbiologist I know) and that there be dramatic tension -without- inadequate lighting. Did pretty well with that.

Another, that I'd be doing a noir detective story with lovecraftian elements, but without using any lovecraftian words--so no squamous, no rugose, no cyclopean, no shoggoths nor Nyarlathotep--that one turned out very well.

Conversely, the attempt to write a western with no proper nouns failed miserably. Little bit too limiting there.

This year, I'm thinking that I'm going to write an Anglo-Saxon epic--think 'Beowulf'--but I haven't yet decided on the plot nor the limiting factors.