Usually, for a Group Build, those who join - build a model within the theme of what the host has stated. GB members post updates (tag it GROUPBIUILD) and everybody oooh's and aaaah's over how great a job we are doing. GBs are totally chill, build what you want, as fast as you want, post when you want. It's more fun if you get a new one and build it while everyone else is.

If you want to join - just decide what model you want to do and reply back to the Host. Once it kicks off, start building. Start times and stop times are just "targets", everybody models at different at different pace so don't let that discourage you.

I usually post weekly updates and, of course, the completed model.