The Hospital Harem is one of multiple routes in Adventure High.
In it, the Protagonist has enslaved Dr. Tiff and is using her to enslave the other students in the school that comes to the hospital for healing.
To access this route, the Protagonist must start the Redemption Route and then get help from Ms. Rack to enslave Dr. Tiff.
Once done, speak to Dr. Tiff again and do not talk about Ms. Rack - instead, encourage her to enslave any student that comes there. Note that you must have reached floors thirteen and fourteen of the Dungeon for the first two scenes to play out - there is no upper limit, so reaching floor fifteen for the Sigil still makes you able to start those scenes.
After you've done that, just enter Dr. Tiff's room - there's a 1/4th (25%) chance that a scene will trigger, in which she will enslave a female student that entered.
At any point after this scene, enter the room and another scene might trigger in which the Protagonist gets the option to either enslave a nameless NPC or simply cure her and let her leave again.
Should the Protagonist enslave her, she'll be told to bring more people to Dr. Tiff's room to get enslaved, which she obeys to doing.