Okay, maybe I can help since I have BTDT !
I was there with my family in a 2 bedroom. We only had 6 people in the room, and my Aunt came to visit us for a night. We had the room to sleep in and she wasn't going into the parks with us at all. I did need a parking pass for her to keep her car in the parking lot overnight. Went to concierge desk in the lobby. Told them about aunt and asked for parking pass. They said no problem, I did not need to add her to the room and they gave me the pass. I gave her a room key from one of my kids and did not have to add her to the dining plan or even register her to our room.
A few weeks ago, same Aunt joined my DH and I for a night in our studio with concierge access for one night. I asked again , different hotel, for parking pass. Told it wasn't need since I had her car parked valet with my
DVC card. Didn't need to register her and could just take her with us to the lounge with our key card or give her one of ours to use for her to go alone.
I was told as long as we are within the allowed number for the room then it wasn't a big deal to have an overnight guest.
Same Aunt will be visiting us again in 2 weeks while we are there. We will be on the dining plan when she joins us on Friday and stays til Sunday. She will not be going to the parks with us, she will have one of the kids keys to get in and out of the room and will mostly just hang around the pool and join us for a meal or two, maybe. I will not be adding her to the meal plan as I didn't the other times.
I will be there from the 3-10 and she will be there to sleep mostly from the 5-7. I am staying wihtin my allowed room guests. She isn't getting any perks of EMH or even her own room key. She most likely won't even be going into the parks with us at all. I am going by what has happened in the past and what I have been told I can do.
OP, the only problem I forsee is getting your drop-in a room key. Registerng them as a guest will most likely apply them to all the rules you promised to uphold in the contract. They will have to buy the same dining plan for your entire length of stay. They will get a key to allow them to use EMH. Un-registered, they do not ahve to buy the plan and cannot do the EMH. But, they can still join you for meals and pay their own way (cash or CC) for each of them.
And yes, they will not be able to use ME if un-registered.