Hi Stu,
Seven friends and I have been offered a couple of weeks in a gallery in Cork. This is the first time we have shown in a gallery (we had an exhib in a cafe and on college walls before), and we have been agonising over the same thing. Trying to get 8 people to decide on a theme was impossible, and in the end we gave up. The advise we got was -
1. Make sure the prints are of very high quality (all darkroom b+w)
2. Frame them well.
3. Although no overall theme, each person will make sure their prints (5 each) work well together - loosely themed I suppose
4. If selling, offer them in batches - ie 1 of 5, 2 of 5, etc. No need to print them all up (mightn't sell any!), but keep good printing notes, so you can print subsequent prints identically
Most importantly, enjoy it. It's a steep learning curve, but it's great to hear of so many people exhibiting film photography!
Rory