newgrp changes the current real group ID to the named group, or to the default group listed in /etc/passwd if no group name is given. newgrp also tries to add the group to the user groupset. If not root, the user will be prompted for a password if she does not have a password (in /etc/shadow if this user has an entry in the shadowed password file, or in /etc/passwd otherwise) and the group does, or if the user is not listed as a member and the group has a password. The user will be denied access if the group password is empty and the user is not listed as a member.
If there is an entry for this group in /etc/gshadow, then the list of members and the password of this group will be taken from this file, otherwise, the entry in /etc/group is considered.
id(1), login(1), su(1), gpasswd(1), group(5), gshadow(5).