Is your road association a "Statutory" road association formed under 23 MRS sections 3101-3104? If so, section 3101 states,
"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, ..."
And,
" 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..."
I would take that to mean that the owners of all parcels benefited by the road are required to be members - especially in this case where you say there is a deeded right.