To add another admin user to your event page group, log in using the email address associated with your event organiser account and visit Events > Settings*.
Under Manage event users, click Add user and add the first name, last name and email address of the new admin user, then click Save.
When done, remember to inform the person that they'll now be able to log in as administrator of the event.
Note: This will only be visible if you have previously created an event page.