usera$ grep groupa /etc/group
groupa::1000:usera,userb

usera$ touch foo
usera$ chgrp groupa foo
usera$ sudo chown userb foo
usera$ ls -l foo
-rw-rw-r--  1 userb ... foo

usera$ echo Hello World > foo
usera$ ls -l foo
-rw-rw-r--  1 userb ... foo