If they're going to just be coming in during your reservation - there will be no "checkout/checkin" process when they arrive.
Like MS said, if you won't be there when they arrive, then you will need to add their name on the reservation, so they can pick up a key at the front desk, or, even better, when "you" checkin, tell the front desk that the guests will be arriving (tell them the specific day and approximate time), and ask them to place a key at the desk for them. I would request that charging priveleges NOT be placed on their room keys.
And the credit card situation is right. You need to forewarn your guests that <u>your</u> card will be on the room for any incidentals. They need to realize that anything they charge to the room (phone calls!) will be billed to YOU, and you will expect to settle up when they return home. I don't think you can change credit cards mid-stay. But, check with the front desk. You may be able to go in and "settle up" the night before you leave. But, your guests will need to place a new card on the room when they arrive, or they won't be able to enjoy the benefits of "express checkout." Maybe someone else has more experience with this type of situation...
The bill for the week will be placed on the doorknob to the room on checkout day. You need to remind your guests to return that to you or, maybe, you can make some sort of arrangement when you check in to have that mailed to you, directly, instead.