The most famous name is probably Dora Richter. She worked directly with Hirschfeld and she's usually regarded as the first known person to receive a vaginoplasty (in 1931). She had a very difficult life and disappeared off the face of the Earth in 1939 though.

Unfortunately though history wasn't great at recording the names of the various queer people who were clients at the institute. For one, most of them wanted privacy. 1930s Germany wasn't exactly the greatest time to be a publicly known queer person, so many of the records used pseudonyms or abbreviations of names. A lot of the recorded names are just a first name, last initial, and address, for instance. Second, the Nazis burned the Institut's archives, so much of the notes/research are lost.

I should also point out that Hirschfeld himself was a gay man. His partner, Karl Giese, was the Institut's head archivist and he was killed by the Nazis at the Lodz ghetto in 1942. Any amount of research you do into the history of this place will probably fill you with terrible sadness and I'm sorry. They were really ahead of their time, reaching near modern levels of understanding of queer people, and yet the entire thing was stomped to death by fascist scum.