virtually all the major casino/hotels have people checking for key cards before you can enter the elevator lobby, especially in the evenings.   It is almost SOP for gals to come down and escort their guests up in the elevator.  Often times she will get two room key cards, and hand one to her guest so they look like a couple who are registered at the hotel.

At least, that's how I've always gone about it there.

 
Maybe others have another take on this.

To the Wynn (north end) will work. All are busy and large enough you won't have to worry. Some hotels are more escort friendly than others, but you shouldn't have any issues. The hotels in City Center are very popular.