The first thing that pops into my mind is Sunshine v Brett. There is a link to it on the MARA Resources page. I’m not an attorney and therefor cannot interpret law or give legal advice. What follows is just my inexpert opinion.
It appears to me that in Sunshine v Brett, where not all property owners were properly notified when the association was formed, the Court did not invalidate the entire association but did declare that the assessment could not be enforced against the person who complained about the error. He was NOT one of the ones who had not been notified. In his dissenting opinion, Justice Alexander said that since Brett had agreed to the initial terms of the road association, in which they agreed not to charge undeveloped lots, he was not injured by the fact that the owners of those lots were not notified, and had no real grounds for complaint. But that’s only the dissenting opinion. The rest of the Court determined that the road association could not collect from Brett:
“...the association failed to comply with the statutory requirement that it send notice of the initial meeting to all owners of parcels benefited by the private road. As a result, the association was not eligible to make assessments for the years in question.”
This sounds like the association could not properly have collected from anyone. But I never heard anything about the other members of the association demanding refunds. The dissenting opinion would support the position that anyone who agreed to sign on under the terms that undeveloped lots would not be charged, would not be injured by those lots not being notified. But your question is a bit different, in that it appears your road association’s formula should have required the omitted person to pay, which could have reduced everyone else’s dues somewhat.
Sunshine v Brett also refers to Tisdale, 2003 ME 68, ¶ 23, 822 A.2d 1136. In that decision, the Court said that the road association could not properly bill a member for years when they had collected from other members but were not yet formed as a statutory road association. I think that decision is applicable here, although the situation is a bit different. I would say that the road association cannot charge the omitted member for years when the association made no attempt to notify him. If they notify him now, and begin billing him in the NEXT billing cycle, hopefully that owner will agree to become a paying member. If not, then the best remedy might be to re-boot the association - start over as if it were a new association, with a Notice from a Notary, sent to EVERY owner of benefited property. If you have any unpaid members, the new association would not be able to pursue claims against them before the date when the new association came into being.
One thing you didn’t mention - does the lot in question have other access? If so, the Board may have thought this lot didn’t need to be a member. However, it sounds like they are using the road for access and are benefiting from maintenance and snow removal, so they should be a member.
I would recommend trying to avoid a lawsuit over it. It’s cheaper to be a neighbor than to be an enemy!