How would you differentiate between voluntary organizations and factions?

I would say that the distinguishing factor between the two lies in their motivations. Voluntary organizations are typically associations that unite people for the purpose of the common good. Essentially, people band together for the collective good for all. Factions on the other hand, are formed for the means of pursuing self-interest rather than the good of all people. James Madison in his Federalist No. 10 papers stated that factions are "groups of citizens with interests contrary to the rights of others or the interests of the whole community". In today's society there are several examples of voluntary organizations: Elks club, Lions Club, Eagles and factions: Labor Unions, Political Factions and even groups that fall under both categories such as the NRA for example. I hope this helps :)