If you want to stay in the Altzomoni Lodge (Albergue), apparently you are supposed to get the key in Amecameca (though one online source says Paso de Cortes?).
We didn't know this and got all the way up there with no key and we left the tent down in Amecameca.
We stayed at the hut at Paso de Cortes since the Altzomoni hut is easily booked out. For logistics, we used a local company from Mexico City who offer transportation, lodging at Paso de Cortes and food+water (before your climb, lunch for your backpack and warm lunch at your return) for a really good price and I'm happily passing over their contact: nomadatravelmexico at gmail.