Try hand walking them in to the field and letting the horses smell them and then put them in a paddock beside the field there suppose to be in so they can get away from the other horses if they need too.
I completely agree with what Lucy said, but along with that, make sure that whatever pasture your horse will be going in is big enough. Your horse needs to be able to get away from the rest of the herd if necassary.
I agree with Calista that it's sometimes useful to introduce horses over a fence, since they cannot kick or injure one another and can start deciding who is higher in the herd pecking order.
However, horses fighting for herd rankings is completely normal and injuries are pretty much unpreventable, unless you wrap your horse in bulletproof armour and cotton wool. Being a part of a herd, horses learn about pressure and release and learn how to be submissive to a 'higher' horse. They learn to fend for themselves and can be, well.. horses.
It's understandable that nobody wants their horses to get hurt, but it's a horses instinct to want to be higher in the herd pecking order and the only way to do this is by fighting it out. Worrying about this is the worst thing you can do as this results in people keeping their horses cooped in a stable to have no social life whatsoever because they don't want their horses to be hurt and would rather keep them in a box rather than leaving them be in a field with their buddies.
I've seen my own mare get kicked, bitten and threatened by other horses. Am I bothered? No. Because I know that she's a horse and she has her own mind. She knows what's best for her when she's in a field with other horses since, at the end of the day, she's the horse here, not me.
First introduce him to friends over a safe fence, then if all seems safe and quite release him with the others, also try pair him with one other horse, so it seems as if you don't just throw him in the deep end ☺️ And expect to find a few kicks and bites at first, he might just be finding his place in the herd but watch out for bullies!