If doing indoor group shot, lens choice (especially for brightness) is secondary. It's more about getting sufficient DOF, lighting, and man management. No point in getting the front row sharp, and middle/back row all out of focus. Make sure you have the right combination of lighting, focal length and get it done quick before the party/guests start to moan/look grumpy.
To answer your question, I would agree, if you don't know what you want, then a prime is going to bite your ass. use a standard zoom. 17-55mm or 24-70mm as suggested should give you enough room to manoeuvre.