Its so easy, all you have to do is go to the events page choose your event you wish to book on, click on your choice,
This will open the booking page start typing your info and it will load all your details automatically. Look further down the page and you will see a tab labelled "I am bringing a plus one", click that and a further box will open for you to put you plus one details in.
Press "book now" and it's all done. You will receive a confirmation Email saying that you are booked.
Should you wish to amend the or delete the booking click the "amend/delete" tab and it will show you the organisers details to contact so that they can amend or cancel your booking.