In order to tell you for sure, I'd need to see exactly which road and lots are involved, and how they relate to each other.  Could you send me a private message at roadways@juno.com?   (Disclaimer - I am not an attorney, so I cannot give legal advice or interpret law.  What I say is based on 40+ years of experience with Maine road controversies.)

In general, if the subdivision is a new division of a lot that was considered a "benefited" parcel when the statutory road association sent out its original Warrant, then I would think it would already be included.  Any division of a lot that was notified at the formation of the road association would by necessity already be obligated to pay an assessment to the association due to being a benefited lot.  Creating a subdivision would create more traffic, so each lot in the subdivision would need to pay an assessment at the same rate as other similar lots as defined by your assessment formula.

On the other hand, if the parcel being subdivided was not among the lots notified and made a member when your Warrant for the original meeting was sent out, then the question is whether or not it is in fact a "benefited" lot.  If not, then it may be a "stranger to the deed," which could cause problems if it is accessed over your association road when it should be accessed from elsewhere.  If it was a benefited lot but was not included when the original Warrant went out, your association could have serious problems.  (See Sunshine v Brett - there's a link on the Resources page of this forum.)

As the attorneys who speak at the MARA conference always remind us, the best way to stay out of lawsuits is to make sure you follow every requirement of the statutes to the letter.  Keep that in mind as you decide what to do, especially if you are considering amending your bylaws.

I have had two experiences with private roads and added subdivisions.  Currently I live on a road that has a nonprofit association.  Before I moved here there was an additional subdivision approved.  According to history the town approved the subdivision with the requirement they become a member of our nonprofit association and also the new subdivision was responsible for maintenance of their own section of road that extended off our existing roads. This was recorded on the plan and in the deeds.  

In Windham we formed a statutory road association for a 725 ft section of paved road that had 4 other roads that benefited from this paved road the association was going to maintain.  The four other roads consisted of a dirt road with no association, a dirt road with a long standing statutory association, and two new paved roads that were part of a new subdivision approved by the town.  The new subdivision did have in the approved plans and deeds that they had to form an association to maintain their roads.  

We did approach our attorney about have all the roads under the one new statutory association as we drew up the bylaws. The main concern for residents along the 725 ft of paved road was paying for maintenance on the roads where they received no benefit of ingress or egress.  

Our attorney suggested having all the roads under one association could work by each road having their own road commissioner who would prepare their annual maintenance budget.  The treasurer would have to compile all the commissioners inputs for the budget then calculate out what residents/owners would have to pay based on what street they lived on.  While this could be done easily with a spreadsheet to calculate the different assessments it was felt the arrangement would be too complicated to administer over the long run.  We ended up with a new statutory road association for the 725 ft of road leading in and the remaining roads were left to form their own association if it did not exist already.

Hope some of this background helps.