To buy more than one ticket, you must select the amount of tickets you want before continuing with the purchase.
If the experience you want does not allow you to increase the number of people, it means it only allows a maximum number of tickets per user. In this case, your friends will also need to download the app. Besides, if they enter your code before any other code, you will get £5/$10 for the first purchase of at least £2/$2 they make with a credit card or PayPal account for plans that cost £7/$12 or more
Note that it is also possible that tickets are no longer available and that is why you can not select any more. We recommend you to check always first that you can select as many tickets as you want before confirming your purchase.