Preferred Gender Pronouns:
gender neutral: ze/hir/hirs
plurals/singular they: they/them/their
People may also want you to mix and match their pronouns, as in, be inconsistent within the same sentence.
the royal we: we/us/our
Many people contest that plural pronouns (they/them/their) are not grammatically correct. While one might simply say that this is not true, I think it is also important to note that this is something of a moot point. Is your track record with grammar really so perfect that you can’t stand to tarnish it in order to respect other people’s wishes about their gender? I doubt it. If you are a cisgender person, our language was made for you, and you should check your privilege at the door. Then come on in!
The correct way to use the word transgender is as an adjective, not a noun. There is no such thing as “a transgender,” only “a transgender person.” There is some debate over whether the use of “transgender” or “transgendered” is more grammatically correct and unproblematic. GLAAD says “transgendered” is problematic, and many trans people feel the same, but it is also true that “transgendered” seems to be more grammatically correct. Certain communities have a very strong preference for “transgender,” however, so remember to listen and be careful.
On language use:
Language is an ever-changing phenomenon that has to suit the needs of those who use it, and if an identity receives insufficient representation in standardly accepted usage, then grammar is failing in its service to its users, not the other way around. We are constantly remaking language in our own image, and always have been. There are places, in short, when language needs changing, and language about gender is one such place.