groupadd and useradd

Add a group 'friend' with a specific gid 600:

groupadd --gid 600 friend

Create a user with a specific gid 'friend':

useradd -g friend -m username

Add a existing user 'username' to a group 'friend'

usermod -a -G friend username

Change username's primary group to 'friend'

 usermod -g friend username