You can only assign an Event Role to an event's Guests, Workers or Managers once those Event Roles have been created and added to the Event Category of that event.
How to assign an Event Role to an event's Guests, Workers or Managers
Go to the Event page
In the Event Managers panel, a dropdown list appears below each Event Manager. Use it to assign an Event Role to that person. There is no Save button, the information is saved as soon as you make the selection.
In the Event Workers panel, a dropdown list appears below each Event Worker. Use it to assign an Event Role to that person. There is no Save button, the information is saved as soon as you make the selection.
In the Event Guests panel, a dropdown list appears below each Event Worker. Use it to assign an Event Role to that person. There is no Save button, the information is saved as soon as you make the selection.