(1) Within 20 days from the first day of posting of the notice of unit determination proceedings, any labor organization or group of employees may file a petition to intervene.

(2) The petition shall contain the name and address of petitioner.

(3) The petition shall be accompanied by proof of interest consisting of authorization cards, or copies thereof, which have been signed and dated within six months prior to the filing of the petition representing 10 percent of the employees in the unit.

(4) The petition to intervene shall conform in all other respects to the requirements for a petition for new unit determination and election.

(5) The division shall serve a copy of the petiton to intervene upon all other parties.