I suppose you mean by 'coordinates' floor and room, not x and y.
There are only 4 floors and 2 rooms, total 8 possibility's. Can as well create them all 8 at once, in 2 little nested loops.
Give them an id, floor and room instance variable, and pick them randomly.
I dont understand your code.
Example: '&' means for strings what '+' means for numbers. '&' combines strings (ads a string tot a string). "a"&"b"="ab"
You never stored floor&room in the array, so you will not find it in there too.
You add 1 to guest number, even if that was an illegal combination.