'man' is your friend!
excerpt from 'man group';
group - user group file
/etc/group is a text file which defines the groups on the system. There is one entry per line, with
the following format:
The field descriptions are:
the name of the group.
the (encrypted) group password. If this field is empty, no password is needed.
GID the numerical group ID.
a list of the usernames that are members of this group, separated by commas.
If you set the 'passwd' parameter then a password will be required.