The maintenance of Event Managers can be accessed from the Managers option on the Event Summary panel More Options Menu. 

Event Managers of the Event will be listed on the Primary report.  

By selecting the Maintain Managers button, users who have Event Manager status and are part of the all the groups which are part of the Event will show as possible options for adding as an Event Manager.

Existing Event Managers will also be shown and can be removed as long as they are not the default Event Manager.