My cat came home from the vet with a pin in his leg, and now the other cats growl or hiss at him. Is there anyway to help them be more friendly with eachother?

The pin in the leg has absolutely nothing to do with why the other cats are growling and hissing. They had to do surgery and they had to clean the surgical site so they used a solution that removes germs AND scent of the cat.

Your other cats knew the cat that left but they don't know the cat that came back. Cats identify by scent. This cat doesn't smell like the cat they know thus they consider it an intruder and they are warning it and trying to protect their home.

It happens every time a cat goes to the vet and has surgery including spay and neuter surgeries. It can take a couple weeks for the surgery cat to regain its own smell. You might try helping things along by rubbing down the other cats then using that same towel on the surgery cat or brushing. Basically transferring other cats scent to the surgery cat so they at least recognize their own scent and make the surgery cat neutral until its own scent comes back. Either that or keep it separate in a different room until the scent returns.

My cats did the same thing whenever my guys had surgery. The vet told me it was a smell they responded to (probably an antiseptic). It took a few days but no one got attacked, they just thought he smelled funny I guess.

Your cat came home with that nasty vet smell and doesn't smell "right". Put him in his own room for now (with food and litter box). Wipe him down well with baby wipes (avoid the surgical area of course). Tomorrow let him out, and see if it's better. If not - repeat. Once that vet smell goes away, all will be find.