Regardless of commas, I've added a bugzilla for this now :-).
https://bugzilla.redhat.com/show_bug.cgi?id=470233
I also see this very old request:
https://bugzilla.redhat.com/show_bug.cgi?id=212939
That one wants to be able to add multiple users,
and running system-config-users would do that
as well.