Is it Islamically permissible to benefit from employee discounts in hotels and sell rooms at a price lower than the advertised rate but higher than the discounted rate?

The contract between you and the hotel is either a lease (ijarah) or a brokerage (samsarah), and both are permissible. In the case of a lease, it is permissible for you to rent a room from the hotel and then sublease it at a higher price, because you own the usufruct (manfa'ah) and have the right to utilize it yourself or through others. However, it is not permissible to lease a specific room before renting it from the hotel, because that would be leasing what you do not own, which is invalid.

It is permissible for you to contract with your friends for a room with specific descriptions and then provide them with what was agreed upon. This is considered a described lease in obligation (ijarah mawsoofah fi al-dhimmah), and it is not necessary that you have already rented it from the hotel.

In the case of brokerage, if the hotel authorizes you to rent the room at a specific price and the remainder is your commission, this is permissible, even if the room's price is unknown and they tell you: "Rent it for such-and-such, and whatever exceeds that is yours."