Option 1. via Attendees manually
Go to Attendees > search for the person you'd like to seat at the session > use the drop-down as shown in picture below > seat the person by clicking assign a session
Note: if in email template "seat confirmation" is automated, the person will receive an email directly.
Option 2. via Sessions manually
Go to sessions > search a session you'd like to add people to.
After finding your session, click Show seat/requests
3. Find the person you'd like to add to the session via the drop-down and click seat
Note: when the email 'session confirmation' is automated (check if button automated in email templates is shown) it will automatically sent an email to the person you seat