I and 3 others are sharing a room. One person booked the room for the 4 of us. 3 of us will be arriving earlier than the one who booked it through the portal.
Will we be able to check-in okay when we arrive before the one who actually booked it?
Depends on the hotel. Most will want a form of payment before releasing any room keys. If you can do this and have the one who booked note who will be showing up the hotel can work with that...and the original booker can go swap out credit cards at the front desk.
Or have them store your luggage and go do something until the one with the keys arrives.
The person who originally booked the room should add your name(s) to the reservation. Not sure if this has to be done through the portal or through the hotel directly, but I would recommend adding directly with the hotel if they are given a choice. If something gets lost in translation when using a 3rd party site/app/program, hotels are more than happy to throw you under that bus if something happens and you didn't communicate with them directly.
As long as your names' are on the reservation, at least one of you is 21+, and can provide a credit card, should be able to check in like normal.
Q-rooms (the housing portal) transfers reservations to the actual hotel approximately a week before the convention. When that happens, I'd just give the hotel a call... the front desk of the actual hotel and not the chain's generic 800 number. They should be able to help you out.
Edit: I'd encourage EVERYONE to call their individual hotel booked through the portal once Q-rooms transfers them. Three or four years ago there was some glitch in the transfer and I was almost screwed out of my room because they were expecting me to check in a day before my actual reservation with Q-rooms.
We didn't have issues with this at the downtown Marriott last year, I just added my roommates to the reservation and they let someone else use a card at check in. I can't remember 100% but I think added them through the reservation on Marriott's site