Since your EEM is tied to LDAP , all users present as part of LDAP would be available as contact in Service Catalog. You can use Report Data Object to query the ca_contact table to fetch contact details. This report object can be used as part of your form to provide the user with an option to select a valid contact that is already present.
In the form you will have to use the lookup or drop down option where you can associate the report data object.
By using this you can skip the validation step. Hope this helps