Pierre Leroux was the other half, along with P.-J. Proudhon, of the mutualist mix, as formulated by William B. Greene. Greene was introduced to Leroux's work by Orestes A. Brownson, and adopted a number of Brownson's criticisms of Leroux's works. Greene's first major writings were, in fact, attempts to come to terms with the thought of Leroux and Brownson. From this perspective, Brownson's most important works were a review of Leroux's Humanity and "The Mediatorial Life of Jesus," both from 1842 - and they're both available now in Corvus Editions.
If you want to understand Greene's mutualism, or want another avenue of approach to the "collective force" stuff that appears in Proudhon, give these essays by Brownson a look. They will feature prominently in the essays in LeftLiberty #2 as well.