In order to create a table and invite a friend click on the Create a Table button:
Then you can create the table that you like:

Click on the Invite Friends button (bottom left)

Click on the empty seats where you want your fiends to sit on

Then choose a friend in the list and click on the Invite button

If you have many friends you have to slide the bar on the right to see them all
or you can enter the beginning of the username you are looking for in the black bar (indicated by the yellow arrow in the image below) and only the usernames that begin with those letters will appear

At last, click on the bottom right button Open Table