My preferences:

--maximum of 6 participants
--I'd rather participate a group that's mixed with regard to format, process, and subject, but if there are interesting exchange groups that want to do something more specialized (probably portraiture or architecture, preferably large format), then I could go with that.
--global participation preferred
--if individuals want critique, that's fine, but that's not really a motivating factor for me
--gallery optional: I'd rather look at real prints