Is it permissible for a real estate office that manages a property for a monthly percentage from the owner to demand an additional commission from the tenants, and are the real estate agents working for it on a commission basis entitled to take a commission from the tenants in the same situation?

Since the work of the real estate office extends beyond mere marketing to what was mentioned in the question, it appears that the relationship between the office and the property owner is one of agency for a fee. It is not permissible for an agent to take a commission without the permission of his principal. Therefore, it is not permissible for you to take a commission from customers without the permission of the property owner. You can agree with him to take it now, seeking his forgiveness for what has passed. If he does not agree, then the past commissions must be paid to him, and you must stop taking them in the future. In this case, the real estate representatives are entitled to a reasonable wage for what has passed, not a percentage of the forbidden commission. You can contract with them in the future for a salary or a percentage of the commission.