When setting up an NIS domain master per FreeBSD's documentation, you
are instructed to make a copy of master.passwd in /var/yp, remove any
system or administrative accounts and make domain-name, which succeeds.

This leaves you with your users' accounts in both /etc/master.passwd and
/var/yp/master.passwd, and if you wish to add a user, you must add to both
for the user to be valid on both the domain master and the nis domain.

The solution is to set the system up as an NIS client per FreeBSD's
documentation, by adding the following lines to /etc/rc.conf:

nisdomainname="domain-name"
rpcbind_enable="YES"
nis_client_enable="YES"
nis_client_flags="-S domain-name,server1,server2"

Remove the users from /etc/master.passwd that exist in /var/yp/master.passwd,
and add +::::::::: at the bottom of /etc/master.passwd.

However, after rebooting with this configuration, the users in
/var/yp/master.passwd do not register on the system.  

ypcat passwd
clnttcp_create failed
ypcat: no such map passwd.byname. reason: Can't communicate with portmapper

Connectivity to rpcbind is allowed through /etc/hosts.allow, and there are
no active firewall rules.

Fix: 

Unknown.
How-To-Repeat: Configure the system per FreeBSD's documentation as an NIS Server, then
add to that configuration the steps required to run as a client of itself.