I have a pop up portal set up to come up whenever a specific event has fees associated with it. Within that portal would it be possible to enter different visitor id's or names?
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.
Retrieving data ...