A few of our local AQS shops have "sew in's" like you describe. One charges $7.00 and the other $10.00. I've never attended either so I don't know if the shop is "open" for business. I know both have several ironing boards for group use as well as large work tables to spread out on (in addition to the tables they are working at).
I think a bonus to a sew-in would be that the shop be open for business ... at least to those joining the sew in. Someone may need thread, a ruler, a fat quarter ... whatever. Why not open the cash register and make a sale too?