group - user group file
/etc/group is an ASCII file which defines the groups to which users belong. There is one entry per line, and each line has the format:
The field descriptions are:
group_name the name of the group. password the (encrypted) group password. If this field is empty, no password is needed. GID the numerical group ID. user_list all the group members user names, separated by commas.
login(1), newgrp(1), passwd(5)