That depends on how your data is structured and the relationship. If you have checked the box "Allow creation of records in this table via this relationship" under your portal table in the relationship dialogue window, you'll be able to add new visitors to the event.
But... I would need more information about you db to help you further.
I do not have the allow creation of records box checked. I am however running a script that is executed with an OnModify script trigger that creates a new record. Let me know if this helps.