The Host of a sync session can Add Users during the active sync review on the fly by clicking on (+) button.
A pop up user list will be displayed. Select the user by scrolling or search with user name.
Click on (+) next to each user.
Once clicked it changes to a checkmark which means user is selected.
To confirm, click on Add Users button.
A pop up will appear stating an invitation has been sent to your invitee.
The invited user will receive an email notification inviting them to join the package.
To join the sync review session, click on Open in Core link from the email notification or open the package from inbox .