If you call Central Reservations, they will want you to add them to the reservation, which may charge you an extra fee for the entire trip.
Your best bet is to wait until you're actually at the hotel and talk it over with the Front Desk staff. They'll be able to add people onto your room on a nightly basis, give them their own room keys, etc and only charging you the applicable extra-Guest fees for those particular nights. (I've read about people doing this, but have no first-hand experience)
The bonus with doing that is that your visitors become "official" Guests of the resort, and thus become eligible for Extra Magic Hours. They will not have any of the extras associated with the rest of your reservation though, like park tickets or the Dining Plan.
Just make sure that adding those two additional people doesn't push the number of people in the room beyond the stated maximum occupancy.
-Rob
