Put the herd and the new horse in side by side paddocks so they can sniff each other and do showing off. If they aren't screaming and rearing, after a week I'd put the new one in with the group. Expect at the least they do a few little bites and kicks, they need to sort out who's boss. Horses rarely fight to injure, they fight to intimidate.
With our horses if we are combining groups of horses, we have two paddocks that connect so they can meet each other before we throw them together. Then we put them together and as long as there's no excessive aggression we leave them to sort things out. There's always going to be a bit of aggression because they're going to be sorting out who's dominant.
Horses in the wild follow a heard over a long period before they are accepted into the group, that is why putting new horse into an established heard will drive the new horse away or at least for a month or so specially if there is some strong characters, even the ones that usually wouldn't say boo to ghost start to display unsual behaviour, I think its quite sad to see a horse being treated like this by a heard when they have also had to put up with moving to a new enviroment as well, so ideally personally I think they should be introduced gradully or in smaller numbers / over the fence etc if you can, but not everyone has the choice at some livery yards etc.
I see it done all the time with little hassle, it's how things have been done on every yard I've been at. But to me, all new horses must go through a weeks intro in adjacent field before they are mix in the group :)