Linux for Programmers and Users, Sections 3.31 and 3.32
Change the shell’s group ID. When you first log into the system, your group ID is set to your default group. Any files or directories that you create will have this group ownership. They can later be changed to belonging to a different group using chgrp. However, if you will begin working on a project where you want to share anything created, it may be simpler to use the newgrp command first, so that all new files and directories will be owned by the specified group.
SYNOPSIS
newgrp group
$ groups
tim cmst270
$ id
uid=500(tim) gid=500(tim) groups=500(tim),537(cmst270)
$ newgrp cmst270
$ id
uid=500(tim) gid=537(cmst270) groups=500(tim),537(cmst270)
$ touch foo1
$ ls -ld foo1
-rw-r--r-- 1 tim cmst270 0 Sep 3 13:57 foo1
See also
Now complete Homework 4 - Permissions of Files and Directories.