To sit with your friend, view the event you are both going to and tap Add Friend. Select your friend from your contacts, and you'll be linked together. Your seats will be right next to each other at the event.
If your friend hasn't reserved a spot, they will be invited to grab a ticket. Once they have a ticket, then one of you will need to tap Add Friend again.
Please note, maximum group sizes are based on the number of available consecutive seats. You will receive an error if you exceed the maximum group size.