Hi Diane,
If an invitee has already registered for an event and they need to add additional Sessions they are able to modify their regsitration and add in additional Sessions available to them.
First modifcation needs to be enabled in the event by going to Website & Registration>Registration Settings>Basic Settings>Edit>Enable Modifications.
The process regsitrants can take is to log into their "My Registration" Pages by going to the website>click on "Already Registered">Enter in their Email and Confirmation Number>Modify>Add in Sessions>Finish. Invitees could also be sent an email through Cvent that contains the {[E-MODIFY REG LINK]} Data Tag which will bring them directly to the same page to fill out their Email and Confirmation Number to access their "My Registration" Pages as well.
Best,
Lauren