Domain Matching allows you to use a office's email domain in order to streamline the Contact Creation Process on the portal for new members. A contact affiliation record will be created when the contact verifies their email so that they are associated with their Parent Account from the get go. Members that have a Account Portal Role that has the "Manage Roster" setting toggled to Yes can configure Domain Matching on the portal.
Make sure the Service Configuration Property for EnableDomainMatching is toggled ON. For more information on this see the Service Configuration Properties Portal Admin article that is also apart of this Manual.