group

The /etc/group file is a text file that defines the groups on the system. There is one entry per line, with the following format:

group_name:password:GID:user_list

The fields are as follows:

group_name

the name of the group.

password

the (encrypted) group password. If this field is empty, no password is needed.

GID

the numeric group ID.

user_list

a list of the usernames that are members of this group, separated by commas.

/etc/group

As the 4.2BSD initgroups(3) man page says: no one seems to keep /etc/group up-to-date.