May I respectfully note that "transgender" is an adjective that describes a noun, like "transgender woman," or "transgender man," or, of course, "transgender person."
Saying "she's a transgender" is like saying "she's a cute," or saying "they are transgenders" is like saying "they are beautifuls." Or "she's transgendered" like "she's prettied."
This may seem a small distinction to some, but a gender slur to others.
Peace