Another thing I want to suggest is that if you do take her inside either test her to FIV/FELV first or keep her seperated from the others and make sure you keep a very strict cleaning regimen. As in washing your hands EVERY time you move between her and your cats as you wouldn't want to infect your cats with any potentially fatal disease. Perhaps you can take her to the vet beforehand to make sure all is well? If not I would suggest a Humane Society or try to contact a Feral Cat Coalition close by, or perhaps they have a Stray Cat Alliance near you as well.