Although the portal will force the user to login, it does not prevent that user from registering someone else who is not in MinistryPlatform. When this happens, re-assign the Default Participant using the Assign Participant Tool.

This can be accomplished through a Process. MP comes with the "Form Response Submitted" Process which notifies the Primary Contact of the Form when a response is submitted. If the default Process does not fit your needs, you can always create a Process that does exactly what you are looking for! For more information, see Processes.