Sorry, "cooperative development of software by people who aren't project members" did come way before the GPL, and is quite separate from copyleft. By 1984 the newsgroups comp.sources.* were alive and thriving (I did get some programs from them, and probably shared a few patches there too). The very first organized source code sharing I could unearth was the SHARE group of IBM users, around 1955. By 1975 there was a SIG TAPE in DECUS (DEC USers) which gathered programs from attentants to its anual event and copied all to the tapes which everybody then took home. AFAICS, by then it already had a longish history.
It is true that large-scale cooperative development really started to work in the mid 1980s, but because networks started to reach interested parties, not because somebody came up with some particular license idea (as important as it turned out to be).
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