Any particular reason why you need a prime for your group photo? A price range of what you would consider as cheap would be useful for folks to recommend any lens to you. Your lens mount too.
Frankly, off the top of my head, I can't think of any 'cheap' (below $500?), very sharp, prime, wide-angle lens.
A wide-angle zoom, stopped down to a small aperture might be more versatile and practical. The Sigma 18-50 DC is acceptably sharp when stopped down, cheap at less than $200, and very small and lightweight. It can only be used on a digital body though.
Can also consider the Sigma 20mm prime.