Where I board, new horses are put into an isolation paddock for about a week to 10 days to be sure they are not sick. From this place they can see many horses around them. Then, if the paddock is available, they are put into a paddock that abutts onto the main herd's pasture. They can sniff over the fence. Then, after a day or two there, they go into the main herd.
Some people advocate putting the new horse into a smaller pasture or paddock with only one or two other horses at first (horses taken out of the main herd) then putting the three of them back into the herd all at once. This allows the new horse to find some herd mates before he gets thrown in witih the large group, where he might end up being ostracized and picked on a lot.