Online crowds are a promising source of new innovations. However, crowd innovation quality does not always match its quantity. One way to improve quality is to enable experts to provide personalized feedback. However, this scales poorly, and may lead to premature convergence during creative work. To deal with these issues, we present IdeaGens, a crowd ideation system that adapts expert facilitation, a successful strategy for improving collaborative creativity in face-to-face brainstorms, to crowd brainstorming. In IdeaGens, experts monitor incoming ideas from the crowd through a dashboard, and offer high-level "inspirations" to guide ideation towards interesting solution themes. In a randomized controlled experiment, crowd workers who receive facilitation through IdeaGens generate significantly more creative ideas that unfacilitated crowd workers.