Statement of Purpose
A Notice of Incorporation or Statement of Purpose (sometimes "Mission Statement") is a written document to act as a legal "contract" with the world-at-large as to your intentions with whatever it is that you are creating. It is the foundation on which you build something. It is the first step to setting up your collaborative (or sometimes other) organization. It forms the organization's core "DNA" that will allow it to thrive and grow. As such, if the interested parties can't agree on this document, there's no use in spending many resources because you will eventually fragment into various factions of power destroying the community and/or wasting those resources before you can even get sustainable.
While there may be a tendancy to dwell on various concerns and obsessions to "square everything with the law" and such, it behooves you to understand the purpose of the law before you capitulate and translate too much of your intentions into legalese. Nonetheless, be sure to state what services you are providing in exchange for any money or property you receive, so it is clear what is implied with such exchange.
The issue of taxes and such is best addressed by stating what your intentions are clearly with regard to acquiring (and perhaps accumulating) money. Anytime you use money made by the State, you engage its legal domain, which is to say: its purpose. It will likely want to have some relationship to you, particularly if you are using a lot of it, or going against said purpose.
Note, that the only real difference between a corporation and a non-profit organization is that the latter's intentions are to re-invest income back into the community (laterally or towards the Purpose), while the former goes to shareholders (upwards).