How to Introduce a New Horse to the Herd
After everyone gets used to one another, separated by a fence, the horses can be put together. Just because the horses were right next to each other and had plenty of time to sniff noses doesn't mean there won't be any squealing, biting, kicking, or galloping around when the new horse is put in with the others, but chances are, the initial excitement will wear off a bit quicker and the herd will settle into its new structure.
Add One and Then Another
Another way to introduce a new herd member is to turn the newcomer out with the others,
one at a time, and add in more. This gives the horse time to bond with one or two members at a time, with the leader added in last of all.
You'll know your horse has been fully accepted into the herd when they all graze together and the bickering has stopped. There's always a small risk of one horse injuring another, even well after the introductions are over. The chasing and other excited behaviors should end as the new horse becomes an accepted member of the herd.