/etc/group

/etc/group   group – user group file 

groupname:password:GID:user_list

/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:

groupname
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 member’s user names, separated by commas.