We're going to ignore the legal issues here and just assume that Spider-man is completely available for the Avengers franchise.
So, how would you want to see him join the team on the big screen? Would you keep Garfield as Spider-man or reboot?
What villains would you like to see?
My take, I'd like to see Spider-man do his normal "Break into the heroes HQ and attack them" as a try-out, and then just tag along (without permission) when the Avengers go on their next big mission.
I'm not sure what villain to use, but the idea of having some big evil mastermind like Zemo or Red Skull with some of Spider-man lesser badguys as henchmen could work.
I'd probably end the movie with Spider-man deciding that he should probably just stay on reserve or something, as Avengers stuff is a bit bigger than he's used to.