An entrance to a hotel can't be _in_ a hotel, as that is used to refer to something _inside_ the hotel. 'the bar in the hotel', 'the lift in the hotel'. Your sentence could be written exactly the same as 'I'll meet you at the hotel entrance.' 'at the hotel' is quite different. You wouldn't say 'the entrance at the hotel' to refer to a hotel entrance. It would be something else -e.g. 'I'll meet you at the entrance to central station at the hotel' - meaning the station entrance that is located at the hotel.

This is purely semantics--there are many different ways to say this phrase. Generally, we would say the entrance to the hotel, because it hints to the fact that the entrance leads INTO the hotel. However, you could also say "of the hotel" or "at the hotel" (though at the hotel is a little vague, because there may be more than one entrance in the vicinity). For the reception desk, I would say "I'll me you at reception" and that would be clear enough, though you could say "I'll meet you at the reception desk inside/at/in the hotel". They all work! Hope this helps!