Here is what the statute requires:

23 MRS 3101 (2).  Call of meeting.  When 4 or more parcels of land are benefited by a private road, private way or bridge as an easement or by fee ownership of the private road, private way or bridge, the owners of any 3 or more of the parcels, as long as at least 3 of the parcels are owned by different persons, may make written application to a notary public to call a meeting. The notary may issue a warrant or similar written notice setting forth the time, place and purpose of the meeting. Copies of the warrant or similar written notice must be mailed by means of the United States Postal Service to the owners of all the parcels benefited by the private road, private way or bridge at the addresses set forth in the municipal tax records at least 30 days before the date of the meeting. The notice must inform the owners of the planned meeting's agenda and specify all items to be voted on, including, but not limited to, all proposed budget items or amendments that will determine the amount of money to be paid by each owner pursuant to subsection 5. Subsequent meetings may be called in the same manner or by a commissioner or board appointed at a previous meeting pursuant to subsection 5.  

The statute doesn't even require certified mail - it just says USPS, so you should be fine on that account.  You sent the notice, and it was their choice to refuse it.  Just make sure you have met all of the other requirements - having a Notary call the meeting, mailing notice to the owners of every parcel benefited by the road, and the Notice to include all items to be voted on including the proposed budget and amount to be paid by each owner.  If you have missed even one requirement, you should hold another meeting to make sure you have covered everything.  But as long as you have met all the requirements of the law, if they don't show up they are still bound by what the members decide.