The best I can tell/remember is that groups went through 4 phases:

1.) No groups (earliest  UNIX) [ I personally never used this except in the V0 retrocomputing]

2.) First group implementation (Thompson) [My first UNIX introduction was with this implementation]

3.) PWB 1.0 (Mashey version) [then saw this post PWB]

4.) BSD 4.2 (wnj version) [and lived this transistion]

Each was a little different in semantics. 

As Doug mentioned, many sites (like Research) really did not need much and groups were really not used that widely.   Thompson added something like the Project number of TOPS and some earlier systems.  Truth is, it did not help much IMO.   It was useful for grouping things like the binaries and keeping some more privileged programs from having to be setuid root.

Mashey added features in PWB, primarily because of the RJE/Front end to the Mainframes and the need to have better protections/collections of certain items.   But they still were much more like the DEC PPN, were you were running as a single group (i.e. the tuple UID/GID).  This lasted a pretty long time, as it worked reasonably well for larger academic systems, where you had a user and were assigned a group, say for a course or class, you might be talking.  If you looked at big 4.1 BSN Vaxen like at Purdue/Penn State, etc., that how they were admin'd.  But as Doug said, if you were still a small site, the use of groups was still pretty shallow.

But, as part of the CSRG support for DARPA, there was a push from the community to have a list of groups that a user could be a part and you carried that list around in a more general manner.   The big sites, in particular, were pushing for this because they were using groups as a major feature.  wnj implemented same and it would go out widely in 4.2, although >>by memory<< that was in 4.1B or 4.1C first.   It's possible Robert Elz may have brought that to Bill with his quota changes, but frankly I've forgotten.   There was a lot of work being done to the FS at that point, much less Kirk's rewrite.

But as UNIX went back to workstations, the need for a more general group system dropped away until the advent widely used distributed file systems like CMU's AFS and Sun's NFS.  Then the concept of a user being in more than one group became much more de rigeur even on a small machine.

Clem