Mich. Comp. Laws § 3.973
In order to more adequately address emergencies that extend or exceed a jurisdiction's emergency response capabilities, either without rising to the level of a state or local declaration of state of a disaster or emergency or in the initial stages of an event which may later become a declared disaster or emergency, the state or any of its departments and agencies, or a political subdivision of the state, including, but not limited to, a county, city, village, township, special district, or other unit of local government, or any combination thereof, may enter into a mutual aid agreement with 1 or more units of government from another state that provide for coordination of communications, training, and response to and stand-by for planned events and emergency responses between the units of government. When engaged in training, stand-by, and emergency response in accordance with the mutual aid agreements, emergency responders from outside this state are permitted to provide services within this state in accordance with this act and the terms of the mutual aid agreement. This act does not prohibit a private company or its employees under contract with a state agency or political subdivision in this state from participating in mutual aid agreements for the provision of emergency medical services personnel; physicians; nurses; mental health, veterinary, or other public health practitioners; emergency management personnel; or public works personnel, if the state agency or political subdivision approves the participation and the contract between the state agency or political subdivision and the participating private company permits the participation.
MCL 3.973