As the Event Owner and Event User you have the ability to change a registered attendee's selected sessions anytime before the session begins (those sessions that they selected at the time of completing their registration process).
1- Log into Expo Pass with your credentials, https://login.expopass.com/login, and choose the appropriate event.
2- In the "Sessions" section, select the session that you would like to add a registered attendee in to.
3- Click on the blue box on the far left that indicates the # of current registrants for this particular session.
4- Click on the "Add Registrants" button on top right-hand side of this session.
5- Now type in the first name of the individual that you would like to add, scroll down as needed to find their complete name. Check the box to the left of their name and then the green "ADD" button at the bottom of this section.
6- Congratulations -- you have now added this attendee to this Session.
If you would like to "Remove" a registered attendee to an existing session, please review this article: Removing a Registered Attendee from a Session.