 | 1 | initial version |
To quote the comment at the beginning of /etc/group - this is on Big Sur, but it's been there for a long time, and is probably even still there in macOS Ventura:
##
# Group Database
#
# Note that this file is consulted directly only when the system is running
# in single-user mode. At other times this information is provided by
# Open Directory.
#
# See the opendirectoryd(8) man page for additional information about
# Open Directory.
##
I.e., adding a group puts it in Open Directory's database, not in /etc/group.