Many crowd ideation systems seek to gather scores of ideas from people online. However, this often leads to many bad ideas and duplication. A dedicated facilitator who guides exploration of the solution space is a common and effective strategy for optimizing ideation in face-to-face brainstorming, but has not yet been explored in computer-supported crowd ideation. We introduce IdeaGens, a social ideation system for guided crowd brainstorming. IdeaGens divides the crowd into ideation and synthesis tasks, and enables efficient data-driven facilitation of the crowd’s ideation. This work can inform general strategies for shepherding the crowd to produce better results for complex collaborative tasks.