Ive just asked a similar question in another thread (CP Vilamoura),
But during some of my reward stays that I have made, these additional ''extras'' are being added.

IC Berlin, IC Dusseldorf 2nd person supplement added to reward booking, this was visible in upcoming stays. (Second person supplement is normal in Germany)
By paying for Ambassador status, then cancelling and re-booking the nights, this charge was removed, so paying for AMB was in effect a saving.

Now I have the one night gift cert, which, in the same way I wish to use for 3 persons (2 adults +14 yr old) staying at the IC Lisbon. Booking a standard room (via the redeem free night cert link) for 3, shows a 55 Euro a night supplement for the third person. This rate does not include breakfast. So total cost for two nights was coming in at 700Euro or there abouts inc breakfast. After a bit more playing with different rooms etc, I found booking a club floor room including access to the club lounge was 600Euro for two nights and no supplements. Using the club lounge will save on breakfast cost and fizzy stuff at night..

There are many catches with rewards bookings, so its always best to check and double check and then check again..then post on here...