Isolate them where the rest of the flock can see and hear them but not get near them. Some folks will advise to quarantine them for awhile and observe for illnesses. To me, most adult birds one buys and gets to examine closely won't show illness or parasites upon examination but still may harbor latent viruses. These may not manifest and the birds may still be carriers. When you unite them with your flock you will still get potential exposure.
I free range all the time, so I just throw my new ones in the mix and let them all sort it out. This works fine for me and I have had no problems with this. I have a strong flock with good immune systems.
But you might want to follow the advice of folks who keep flocks in a confined pen who use medicated feeds and medications for illnesses, if this is the type of operation you have.