The SharedPlans formalization of collaboration (Grosz and Kraus 1999) stipulates that collaborating agents must commit to certain decision-making processes, but it does not specify those processes. This paper presents a mechanism for group decision making that may be applied to the decisions that agents involved in a SharedPlan need to make: adopting the initial commitment, selecting a recipe, assigning agents to subtasks, and identifying various action parameters. The paper thus more fully specifies the dynamic expansion of a partial SharedPlan to a more complete plan. The decisionmaking mechanism is represented by a fixed, fully-specified SharedPlan. A set of speech acts and conditions under which those speech acts invoke the decision-making SharedPlans are also defined. The definition of the force of declarative speech acts is based on Searle's notion of constitutive rules (Searle 1998).