Synonyms for Collectivist (same or very similar meaning)
WordNet sense 1 (a person who belongs to the political left):
left-winger, leftistWordNet sense 2 (subscribing to the socialistic doctrine of ownership by the people collectively):
collectivised, collectivistic, collectivized, state-controlledWordNet sense 3 (advocating or following the socialist principles):
socialist, socialisticFrom the ODE community, based on WordNetadd/edit