(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, from 10% of the employees in the proposed unit, which have been signed and dated within 6 months of the date of the filing of the petition. The cards shall indicate that the signatories desired to be represented for collective bargaining purposes by the petitioner.

(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 board shall serve a copy of the petition to intervene upon all other parties.