On newly created mailfiles, I am wanting to add a Group (as opposed to an
individual user) as an ACL Manager.

I tried using the mail-acl-manager-id attribute and using the UNID for a
group called DomAdmin in our system. However, when I check the ACL, it
just adds a user called 'null' as a person/manager to the ACL. I tried
using the mail-acl-manager-name attribute and using the group name, but it
simply created a user called DomAdmin (which doesn't even exist in the
NAB) and made it a Person/Manager.

Is there a way to add a group instead of an individual?