Our prod SharePoint install has rather limited connectivity due to its
network location (isolation), and we think we are having problems due
to this isolation.

What connectivity is needed to add users by domain\username to a
SPS/WSS? If SharePoint can connect to LDAP (port 389), is that enough?

We only have ports 80 and 443 open to intranet, and we are about to
have 389 closed on us. We wonder if we won't be able to add users
after they close it.