You cannot ask your tenants not to bring their guest as it is not unlawful for them to have guests stay in the rental unit as often as they please. If the number of people occupying the rental unit on a continuing basis contravenes health, safety and housing standards required by law, you may serve an N5 notice of termination where you would need to prove the tenant is contravening the municipal bylaws. The tenants can avoid termination if they reduce the number of people occupying the rental unit to comply with health, safety and housing standards within 7 days according to the N5 notice.