newgrp - log in to a new group
Newgrp changes the group identification of its caller, analogously to
login. The same person remains logged in, and the current directory is
unchanged, but calculations of access permissions to files are performed with
respect to the new group ID.
A password is demanded if the group has a password and the user
himself does not.
When most users log in, they are members of the group named
`other'.