Before you begin, apply for the Apple Enterprise Program. After your membership has been approved, you can sign in to the developer portal. See the following links for more information:
Managing Your Team—Describes how to assign appropriate team roles. To complete the following procedures, you must assign the Team Agent role to the user who will create the certificate.
Accessing Member Center and iTunes Connect—See especially the "Managing Your Certificates, Identifiers, and Profiles" section.