Grant,
Mashey and crew basically did most of the original group work as part of PWB. If you look at the Sixth Edition sources and the PWB 1.0 stuff, that is one of the places you will find differences. With Seventh Edition (or I believe as part of the UNIX/TS work that Ken picked up), the Mashey group changes went back into the Research stream. With one of the predecessors to 4.2BSD (it may have 4.1A or 4.1B but frankly I have forgotten) Joy introduced the group scheme we all use today.
Looking at the TUHS archives, unless I'm missing something, 3BSD has groups that appear to be in the modern format:
% ls -l /bsd/3bsd/etc/group
-r--r--r-- 1 root root 44 1980-01-02 22:08 /bsd/3bsd/etc/group
% cat /bsd/3bsd/etc/group
staff:*:10:bill,ozalp
grad:*:20:
prof:*:30:
% find . -name 'chgrp*' | xargs ls -l
-r-xr-xr-x 1 root root 6960 Dec 30 1979 ./usr/bin/chgrp
-r--r--r-- 1 root root 26 Feb 12 1979 ./usr/man/man1/chgrp.1
-r--r--r-- 1 root root 754 Feb 12 1979 ./usr/src/cmd/chgrp.c
-Henry