I'm afraid once a booking has been completed we can't change any details. The best way to bring someone else along is to create a separate booking. All you need is the original seat numbers and you can book the seats next to you.
And if your original booking didn’t include a seat number? Even better! That mean’s it’s an unallocated screening. Simply make a second booking for your extra seats, and when you get to the screen, you’ll be able to sit where you like and float up, up and away together.