Logged in as root, nsswitch group is 'files', no LDAP nor NIS ever in use
here. User logged out and back in, multiple times. No old processes still
running after user logout.

tail /etc/group
[?2004l
netadm::65:
slocate::95:
unknown::96:
nobody::60001:
noaccess::60002:
nogroup::65534:
pkg5srv::97:
chrony::23:
sshd::92:
users::100:
[?2004h[09:11] root@fatalis:/tmp# tail /etc/passwd
[?2004l
svctag:x:95:12:Service Tag UID:/:
unknown:x:96:96:Unknown Remote UID:/:
nobody:x:60001:60001:NFS Anonymous Access User:/:
noaccess:x:60002:60002:No Access User:/:
nobody4:x:65534:65534:SunOS 4.x NFS Anonymous Access User:/:
dma:x:26:6:DragonFly Mail Agent:/:
pkg5srv:x:97:97:pkg(7) server UID:/:
chrony:x:23:23:chrony user:/var/lib/chrony:
sshd:x:92:92:OpenSSH privsep user:/var/empty:/bin/false
jayjwa:x:100:100:jayjwa:/home/jayjwa:/bin/ksh
[?2004h[09:11] root@fatalis:/tmp# usermod -g users jayjwa
[?2004l
[?2004h[09:11] root@fatalis:/tmp# usermod -g users jayjwatail /etc/passwdgroup
[?2004l
netadm::65:
slocate::95:
unknown::96:
nobody::60001:
noaccess::60002:
nogroup::65534:
pkg5srv::97:
chrony::23:
sshd::92:
users::100:
[?2004h[09:11] root@fatalis:/tmp# groups jayjwa
[?2004l
users
[?2004h[09:11] root@fatalis:/tmp# groups jayjwatail /etc/group
[?2004l
netadm::65:
slocate::95:
unknown::96:
nobody::60001:
noaccess::60002:
nogroup::65534:
pkg5srv::97:
chrony::23:
sshd::92:
users::100:
[?2004h[09:11] root@fatalis:/tmp# tail /etc/groupgroups jayjwatail /etc/groupusermod -g users jayjwa
[?2004l
[?2004h[09:11] root@fatalis:/tmp# usermod -g 100 jayjwa 
[?2004l
[?2004h[09:11] root@fatalis:/tmp# tail /etc/group
[?2004l
netadm::65:
slocate::95:
unknown::96:
nobody::60001:
noaccess::60002:
nogroup::65534:
pkg5srv::97:
chrony::23:
sshd::92:
users::100:
[?2004h[09:12] root@fatalis:/tmp#