Eric, people just chose a theme and build models that are related to it, exchanging tips, info and notes. Some Group Build are about the same exact kit (I think the future Motorbike Group Build will), others allow for any kit, any scale provided it fits into the theme (like the future Sherman family Group Build). There's a start date and a deadline, but it's not really important to stick to the dates...
Usually, someone 'test the water' with a message like this one, and see who's interested. I'm not sure this one will ever 'lift off', but it was worth a try.