"I was with Dan (they/them) and Steve the other day. They hadn't brought a poster they needed and went back to the car to get it."
This demonstrates the semantic problem with using "they" as a pronoun: it isn't clear who went back to the car, (1) just Dan or (2) both Dan and Steve. Nor is it clear who needed the poster and who hadn't brought it.
But is improper to do so. The proper way to refer to a person who is present is by using their name.