What one of the owners here is asking, is this; Is it possible for them to point their A records (or something) to our ISPconfig server and then we provide the correct IP address to go to?
Yes. Just tell them your IP address and let them create the dns a-record.
On your server, just create a nwe website for their domain. No dns settings have to be made on your side.
If you want tp provide email for their domain as well, then they have to create a mx record pointing to the hostname of your server as well. You create then a mail domain and mailboxes on your server.
You dont have to run any dns services on your server for that.