We present a model showing the evolution of an organization of agents who discuss democratically about good practices. This model feeds on a field study we did for a few years in France where we followed Non Profit Organizations, called AMAP (a french short food chain), and observed their construction through time at the regional and national level. Most of the hypothesis we make are here either based on the literature on opinion diffusion or on the results of our field study. By defining dynamics where agents influence each other, make collective decision at the group level, and decide to stay in or leave their respective groups, we analyse the effect of different parameters, like size of organizations, on the stability and representativeness of these organizations. The models proves to be robust and we believe is easy to adapt to other context than AMAP. Moreover the article highlights the tension that exists between stability and representativeness in democratic organizations, along with the negative effect of increasing the number of topics to discuss and the positive effect of having openminded members.