(1) A peace officer making a lawful arrest may command the aid of persons 18 years of age or older. (2) A person commanded to aid a peace officer in making an arrest: (a) has the same authority to arrest as that officer; and (b) is not civilly liable for any reasonable conduct in aid of the officer.
Assisting A Peace Officer
Mont. Code Ann. § 46-6-402, under Arrest by a Peace Officer.
Mont. Code Ann. § 46-6-402