Welcome, Blackthistle.
About group builds: I'm quite new here too — my first FineScale "birthday" was earlier this month. And I had the same question. I think I understand it now.
People who join a group build commit themselves to building a model that fits within the framework of the group build. For example, a member might suggest a group build titled "The Battle of the Atlantic". Members who are interested in building a model representing any ship, boat, plane, submarine, or gun that was involved in the battle are free to join by committing to build a particular model or models. Dioramas would be welcome as well. Models that might be included in such a group build could be destroyers like the U.S.S. Reuben James, the S.S. Queen Mary, H.M.S. Hood, H.M.C.S. Sackville, U.S.S. Reuben James, and S.S. Eros; various aircraft such as PBYs, B-24 Liberators, Condors, Seafires, Hudsons, B-17 Superfortresses, H.P.52 Hampdens. Some members might build various types of artillery or even radar sets.
The main goal of a group build is to foster a sense of community among FineScale members, and there is the obvious advantage of being able to communicate with like-minded modellers. An additional value is the incentive to get off the couch and away from the TV; group builds normally include a completion date.
No doubt my inexperience with group builds means that I have left out some important information. FineScale is an excellent source of information, especially for novice builders like myself; before I joined, I hadn't built a model for decades, and hadn't seriously been involved in the hobby since I learned, at about age 15, that girls were softer and more cuddly than styrene model kits!
Bob