First make sure they have rights to add the vault added to their Windows account. In the Archive Configuration utility on you archive server, if you go to tools -> default settings and select the security tab you can grant attach access. For users off the domain you should use the BUILTIN\Users domain.
After that just verify they have a username and password in EPDM and they can use that to atatch to the vault. There is no setting for EPDM to grant them attach access.