Primary and secondary groups

Question

I've got the basics of primary and secondary groups down, but still have some questions I can't seem to find solid answers to:

1. Can many users belong to the same primary group?
2. Can one user's primary group be a secondary group of another user?

Answer 1

1. Yes, they can.

   $ id foo
   uid=1002(foo) gid=1002(foo) groups=1002(foo)
   $ id bar
   uid=1003(bar) gid=1003(bar) groups=1003(bar)
   

   Changing the primary group of user foo to bar which is the primary group for user bar:

   $ sudo usermod -g bar foo
   

   Now:

   $ id foo
   uid=1002(foo) gid=1003(bar) groups=1003(bar)
   $ id bar
   uid=1003(bar) gid=1003(bar) groups=1003(bar)
   

2. Yes, it can be.

   $ id foo
   uid=1002(foo) gid=1002(foo) groups=1002(foo)
   $ id bar
   uid=1003(bar) gid=1003(bar) groups=1003(bar)
   

   Adding user bar to group foo which is the primary group of user foo:

   $ sudo usermod -a -G foo bar
   

   Now:

   $ id foo
   uid=1002(foo) gid=1002(foo) groups=1002(foo)
   $ id bar
   uid=1003(bar) gid=1003(bar) groups=1003(bar),1002(foo)
   

Answer 2

From the perspective of the user, he has a primary group and 0 or more secondary groups.

From the perspective of the group, it has 0 or more members.

A group that is the primary group for one or more users can be both a secondary or primary group for other users.