Although the Portal will force the person to log in, that person can still register someone else who isn't in MinistryPlatform. When this happens, reassign the Default Participant using the Assign Participant Tool.

Use a Process. MinistryPlatform comes with the Form Response Submitted Process, which notifies the Primary Contact of the Form when someone submits a response. 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.