User Groups and the /etc/group file

Hi,

	I'm confused, Created a user using

useradd -c user_full_name -d /dev/null  -g GroupA -s /sbin/nologin bob_t

I see in the /etc/passwd file

bob_t:x:100:1000:user_name:/dev/null:/sbin/nologin

Where in /etc/group 

GroupA:x:1000

The question is, how come bob_t is not included into GroupA in the
/etc/group file?

Is the /etc/group file only for supplemental groups which the user bob_t
is also associated with? (and GroupA is the user's primary group and
thus no need to be included?)


using the command line parameter usermod -g somegroup username
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