Mont. Code § 76-15-802
Petitions for including additional territory within an existing district may be filed with the department, and the proceedings provided for in part 2 in the case of petitions to organize a district shall be followed in the case of petitions for the inclusion. The department shall prescribe the form for the petitions, which shall be as nearly as may be in the form prescribed in part 2 for petitions to organize a district. Where the total number of qualified electors in the area proposed for inclusion is less than 10, the petition may be filed when signed by a majority of the qualified electors of the area, and in that case no referendum need be held. In referenda upon petitions for the inclusion, all qualified electors within the proposed additional area are eligible to vote.
§ 76-15-802, MCA