  The spontaneous formation and subsequent growth, dissolution, merger and
competition of social groups bears similarities to physical phase transitions
in metastable finite systems. We examine three different scenarios,
percolation, spinodal decomposition and nucleation, to describe the formation
of social groups of varying size and density. In our agent-based model, we use
a feedback between the opinions of agents and their ability to establish links.
Groups can restrict further link formation, but agents can also leave if costs
exceed the group benefits. We identify the critical parameters for
costs/benefits and social influence to obtain either one large group or the
stable coexistence of several groups with different opinions. Analytic
investigations allow to derive different critical densities that control the
formation and coexistence of groups. Our novel approach sheds new light on the
early stage of network growth and the emergence of large connected components.
