Several ways you can do it, most depend on the ages and numbers of the new ones.  No matter what, it's a good idea to keep the new ones isolated for a time to make sure they're not sick, then, if you have room to 'split' your run, or add  second run along side the old one where the new one's can hang out for a while so they can 'meet and greet' through a fence.  If you can't do that, or if it's just a couple, put the new ones in a crate or cage inside the pen so they can meet each other.  after several days or a week or so of that, you can put the new ones on the roosts at night when they're all asleep and in their nightime stupor.  I swear hens go into mini-coma's at night!  
You'll need to watch them though, some pecking and squabbling is normal, but it CAN get out of hand so be there first thing in the morning to act as referee in case they get vicious. (cause they can)