About Mixed Ink
Mixed Ink is a private company offering collaborative writing services. Their services are mildly unique in that they have devised a way for many editors to work on a document together, though this type of technology is present in several other products. Mixed Ink is special because it offers little else besides this collaborative writing feature, and attempts to make it user friendly for authors.
P2P and Mixed Ink
Mixed Ink could be used as a tool to review a single document and to move that document towards a final draft. In this sense it is very nice, and there's a chance that the service will offer at least a couple of people the opportunity to work together on a few documents before the Freemium model tops out and starts to charge for services.
If you are looking to access Collaborative Authoring tools there are a lot of offerings out there. If you plan on using your documents in a voting system you could take a look at the public domain offering from Votorola.