Keep her separate for at least 2 weeks. A month would be better. You need to be sure she's not carrying disease to your flock. Check her thoroughly for parasites and deworm her. After 2-4 weeks, put her in a dog crate, cage or something right next to your chicken run and let them get to see, hear and smell each other for a week. Then you can put her with them but keep an eye out to make sure the others are allowing her to eat and drink. It's always a good idea to have more than one source of food and water so all birds can eat/drink without being driven off.