While I respect Ray's opinion, I myself am not a fan of section 3121, for various reasons that I need not go into here. But for one thing, while it does give you the right to sue a non-payer, it gives you no other enforcement option. A Statutory road association, on the other hand, gives you the option of a Notice of Claim, which is much simpler. If the non-payer wants to sell their land or wants to get approved for a loan, the Notice of Claim can be very effective. But if the person still does not pay, you can then go to Small Claims Court if necessary.
Section 3121 was intended to be a default when there is no road association or maintenance agreement, and the default position is that everyone pays equally. As I see it, if you do have any road association or road maintenance agreement, section 3121 doesn't really apply except to say that property owners must abide by that agreement. If there is no road association, section 3121 dictates that each owner pays an equal share. Section 3101, on the other hand, allows the road association to come up with a formula that works best for them, which may or may not mean equal shares.
Without having seen the deeds of the two properties on the main road, I'm not sure how either statute would apply. They WOULD need to be notified of your initial meeting, as required by 23 MRS section 3101, paragraph 2, which says:
"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. ... 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 ..."
It sounds as if those two parcels would qualify as "benefited" parcels, ALL of which must be notified of the initial meeting (and probably of other meetings as well). Usually, it would then be up to the members of the association to come up with a "fair and equitable" formula for assessing dues. Some associations decide that properties that have other access do not have to pay, or pay at a lower rate, so long as they do not use the private road. Others decide that because the private road provides access to the back of those lots, and is available if they wish to use it, that they pay like everyone else.
In your situation, it sounds like the deeds of those properties require that they pay an equal share. Whether or not the association can decide that making them pay or not pay is "fair and equitable" would be a question for an attorney. If everyone else in the association thinks they should not pay, then who would there be to object? But if even one of the other members thinks it's unfair that those two do not pay, that could cause problems for the association. It's definitely a sticky situation.
Finally, there is no requirement that every owner "sign the agreement going to the registry of deeds." For that matter, there is no requirement that a road association file anything with the registry of deeds, although doing so may make it easier for buyers when a property in the road association changes hands. So no, refusal of the LLC to sign would not prevent you from forming a road association. As a property "benefited" by the road, formation of a statutory road association makes them a member whether they want to be one or not, and obligates them to pay their assessed share.