Linux man pages : newgrp (1)
NEWGRP(1) Linux Programmer's Manual NEWGRP(1)
newgrp - log in to a new group
newgrp [ group ]
Newgrp changes the group identification of its caller, analogously to
login(1). The same person remains logged in, and the current directory
is unchanged, but calculations of access permissions to files are per-
formed with respect to the new group ID.
If no group is specified, the GID is changed to the login GID.
Originally by Michael Haardt. Currently maintained by Peter Orbaek
Linux 1.2 9 October 1993 NEWGRP(1)