This is an important question for others to take note of.
I used the reservation made through Expedia UK for a stay at WDW last weekend. Everything worked fine EXCEPT that at the last minute I wanted to add one person to the last day. I was told that the reservation could not be changed by Disney AT ALL, meaning that Disney could not add another person. I was allowed to add the extra person as a day guest and he stayed overnight with no problem - BUT he did not get a Magic Band, only a card room key.
For us, it worked out fine because we were not using Fast Passes or any of the other things that Magic Bands do (I don't even really understand the whole Magic Bands thing) - we were there primarily for the Princess half marathon. But under different circumstances, it might have been a problem that he could not have been added as a "real" guest to the reservation.