Interesting idea. You guys should post back on the forum with updates on what the group is doing and improvements you've seen or made thanks to the group. I have had a similar idea in the past however my idea was about taking people from different industries in the same area with similar target clientele and getting together on some things. Sharing information with a guy who details high end cars is very little risk to my business but the possibilities are endless.
Just a few ideas I had about the group I was looking to start that could apply to your group.
-Write a newsletter together, each member contributes a small article and it's beneficial to your clients and you without the headache of writing an entire newsletter yourself
-Share other marketing burdens, order large quantities of promotional material with group buying power then individually send them to a printer to stamp your logo on them, you should still save money this way.
Good luck with the idea and bring back some results. It will be interesting to see the idea in motion.