# First I create a user bob (I set the passwd too):
matthew% sudo useradd bob

# Then log in as bob and execute id to see my groups etc.
bob% id
uid=1001(bob) gid=1001(bob) groups=1001(bob)

# In my normal session I add bob to the audio group:
matthew% sudo usermod -aG audio bob

# Back in bob's shell this change isn't seen...
bob% id
uid=1001(bob) gid=1001(bob) groups=1001(bob)

# But I can use su to create a new login shell as a child of the original shell.
# This new login shell will have the new group seting:
bob% su - bob
bob% id
uid=1001(bob) gid=1001(bob) groups=29(audio),1001(bob)

# When bob closes the shell with exit, he goes back to the parent shell, without
# the new group:
bob% exit
bob% id
uid=1001(bob) gid=1001(bob) groups=1001(bob)