When adding hens to a flock, first isolate them for few weeks or so to check for diseases they might add to your existing flock. This includes mites. Also, worm thoroughly so that no new types of worms can be added to your flock.
And, of course, there's the eternal worry about Mareks, though most sites I've read say that it's everywhere.
If you get the hen from the same place as you got your initial flock, I wouldn't worry too much about new diseases.
When the disease-check period is done, many people dump the hen in. If there is a problem (the other hens ganging up on her, etc.) then use an isolation pen so that they can see and hear each other, but not fight. Let this go on for a week or so, then try adding her again.
I personally let my hens free-range (all the time) so I don't have to be very careful about integration once the initial disease-check period is over. But if all of your hens live together in a smaller run, you'll probably have to follow the above procedure.
it actually helps to add two or more hens at a time, thus ensuring a "buddy system." The flocks don't integrate as quickly, but two newcomers are less likely to get beaten up than one is.