change to a new 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 performed 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 (firstname.lastname@example.org).
The newgrp command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.