From your event, go to Guests > Seating Arrangement.
Your guests from your Guests List will appear in a scrollable list grouped by the first letter of the guests' last names.
Simply drag a guest from the list and drop on a seat to assign that seat to the guest. If the destination seat is taken by another guest, that guest will be shifted down to make space for the new guest. You can also drag a guest from one seat to another.
The guests list on the left will automatically reflect the seat the guests are assigned to. You can click on the table number of the guest to go to his/her table.
If the guest has companions, the companions will also be assigned to a seat next to the main guest.
You can seat as many guests into a table as you like. If the table is over-capacity, the extra guests will be displayed in red.
To remove a guest from a seat, hover on the particular seat and click on the "x" icon.