Contact the hotel, ideally the front desk manager. Have them note the reservations so that guests can check in. You should either get an email or fax agreeing to this from the hotel or note the name and title of the person at the hotel who agreed to this.

Do you want your guests to have to give a credit card upon arrival for incidental charges? This should also be clarified with the hotel.

I fact, it might be better to have the rooms set up for direct billing by the hotel, with clearly defined rules for what incidental charges can be added to the invoice and what types of items will be paid individually by your guest. This works well, with absolutely no expectation that you would be there, but OTOH you would lose points/nights/stay credit for the additional rooms.