When a new horse comes into the barn, there are some things you can do to make the transition into the herd a safe one. If you have just one or two on your place, start by stalling them next to one another. Then start riding them together, hauling them together and turning them out in adjacent paddocks. Ideally, I don't like to turn horses out together; anything can happen in just a split second. However, if your facility dictates they go out together, make sure they have ample space. The first time you plan to have them out together, put the new horse in the paddock or pasture first and give him time to explore. Then turn the other horse out to join him.
Throughout this process, if you get to the point that one horse calls to the other when you take one away, you know you are halfway there and the two are likely to become close buddies. If you have three horses total, it is likely two of them will buddy up and one will be the odd man out--much like when three children get together to play.
I had an experience with an orphan foal and a mare that had lost her baby. I wanted the two of them to get together, but had to be careful about how I went about it or I would lose the foal for sure. I started by having the mare in her stall and putting the baby just outside her stall. After the two got acquainted, I held the mare while allowing the foal to nurse. She accepted the orphan for nursing, but I really wasn't sure I wanted to risk putting the two together, unattended, in a stall. Instead, I turned the mare out with the other broodmares and then brought the foal up to do the same. She had started to take off with the other broodmares, but when the foal whinnied, she came right back to him. Mission accomplished! I definitely had to bite the bullet and trust her to take care of him when I put them out with the broodmares. I was willing to take that risk, because I felt comfortable with the groundwork in place.