Each user belongs to a group that is referred to as the user’s primary group. The GID number, located in the user’s account entry within the /etc/passwd file, specifies the user’s primary group.

Each user can also belong to up to 15 additional groups, known as secondary groups. In the /etc/group file, you can add users to group entries, thus establishing the user’s secondary group affiliations.

Note (4 PSARC/2009/542):

his project proposes changing the maximum value for NGROUPS_MAX from 32 to 1024 by changing the definition of NGROUPS_UMAX from 32 to 1024.

The use for a larger number of groups is described in CR 4088757, particular in the case of Samba servers and ADS clients; the Samba servers map every SID to a Unix group. Users with more than 32 groups SIDs are common. We've seen reports varying from "64 is enough", "128 is absolutely enough" and "we've users with more 190 group SIDS).

NGROUPS_MAX as defined by different Unix versions are as follows (http://www.j3e.de/ngroups.html):

Linux Kernel >= 2.6.3 65536

Linux Kernel < 2.6.3 32

Tru64 / OSF/1 32

IBM AIX 5.2 64

IBM AIX 5.3 ... 6.1 128

OpenBSD, NetBSD, FreeBSD, Darwin (Mac OS X) 16

Sun Solaris 7, 8, 9, 10 16 (can vary from 0-32)

HP-UX 20

IRIX 16 (can vary from 0-32)

Plan 9 from Bell Labs 32

Minix 3 0 (Minix-vmd: 16)

QNX 6.4 8