Work dinner for ~12 in West Chester area
I'm not sure what you consider "reasonably priced" but Sovana Bistro in Kennett has a nice room that can accommodate a group of 12. They've got a liquor license but BYO is okay with a corkage fee of (I think) $8. You might also want to look into Foxfire at the Stone Barn, outside Kennett. They're only open for dinner on Thursday through Saturday but they're BYO with no corkage fee, and I do believe they might be able to give you a private room.