a group build is a lot of modelers doing a model of a certain theme. For example the current British/commonwealth group build. The models can be any scale but must be of a suitable vehicle used by britain or the Commonwealth. For example a churchill in brit markings or a chev in aussie markings (for this see rivit review board. 'aussie chev')
they can be OOB or an almost total scratchbuild like the Chev.
Time span is normally about 6 months but this can vary. Pics are presented and voted on by the members for placeings.
hope this helps.