No need to separate. They have probably all been exposed to it already. It is possible that one or more of the new pullets was a carrier, or they have been exposed by wild birds. It can be carried in on shoes, and can be very common in back yard flocks.
Look for swelling around an eye or the face, or any pus in the eye. They can have sneezing, coughing, wheezing, or rattles. A lot depends on if they just have MG or if they get a secondary infection. The earlier you treat with the Tylan 50, the better the chances that more symptoms will not develop. You might want to keep her caged in a dog crate with food and water in the coop with the others, so that you can get her out for each dose of medicine.