Grandpa: One thing I about living in Santa Carla I never could stomach. All the damn vampires.
Max: It was all going to be so perfect, Lucy. Just like one big, happy family. Your boys... And my boys.
Edgar Frog: Great! The Bloodsucking Brady Bunch!
David: Initiation's over, Michael. Time to join the club.
David: Drink some of this, Michael. Be one of us.
David: You'll never grow old, Michael, and you'll never die. But you must feed.
Answer: In the film, being invited in isn't a requirement to enter. It's simply a way for the vampire to render you powerless (i.e. the tests the boys performed on Max).