The pw utility has the option (via pw.conf) to specify the location of
nis (master) passswd file so that user additions are mirrored to this
file.  However, no equivalent feature for groups exists.  In my
opinion, when pw makes a group, that entry should get mirrored to the
nis group file.

Fix: 

a work around is to run a scriotfater pw or adduser to copy the group
entry over. or, atthe risk of some security, use the /etc/group file
as the nis group file.
How-To-Repeat: 
    add "nispasswd /var/yp/master.passwd" to /etc/pw.conf

    add a user via pw or adduser

    see that /etc/master.passwd, /etc/group, and /var/yp/master.passwd
    have all been updated, but not //var/yp/etc/group