I once make a mistake with the drop down box on Hotels.com (leaving it at one not two people) and arrived at the hotel with SWMBO to be told by reception about the problem. Fortunately, since it was clearly a mistake, we all had a laugh about it. All I had to do was agree for a change in price to cover two people for breakfast rather than one and all was fine.
Clearly SWMBO was not impressed that I had ‘forgotten’ her so I had to work extra hard that holiday to make it up to her (as in we went to even more expensive restaurants than usual).
I think that most hotels will be OK about you arriving with an extra person provided you are not in a single only room and you cover any extras. As per the advice above, ideally let them know in advamce.