The most basic question possible...

Think of it like an online book read. Each person posts pictures of their build as they do it, mention any good or bad points about the kit, ask questions if they have any and comment on other people's builds. When you finish you post the finished photos and add the group badge to your signature, then watch as the others finish.  Some people will even include a short history of the subject or why they chose it. Just read through a few of the group builds and you will get the hang of it.

Group builds are also an excellent way to learn new methods of building weathering ect,ect... Most GB participants will post "Work in Progress" pics as they go along....