Example 1: Limit of 6 tickets per purchase. I buy 6 tickets, I invite 5 friends. I show ID at the door and it doesn't matter which 5 friends I bring with me.
Example 2: Same deal, but instead we're allowed 1 "flex" ticket in case someone backs out. Or some proportion allowed as flex.
Obviously this still allows scalping, but it seems less attractive for a scalper to have to attend with the people they're scalping to. Especially if you make 50% of the group show ID.