Learning about a new area of knowledge is challenging for novices partly because they are not yet aware of which topics are most important. The Internet contains a wealth of information for learning the underlying structure of a domain, but relevant sources often have diverse structures and emphases, making it hard to discern what is widely considered essential knowledge vs. what is idiosyncratic. Crowdsourcing offers a potential solution because humans are skilled at evaluating high-level structure, but most crowd micro-tasks provide limited context and time. To address these challenges, we present Crowdlines, a system that uses crowdsourcing to help people synthesize diverse online information. Crowdworkers make connections across sources to produce a rich outline that surfaces diverse perspectives within important topics. We evaluate Crowdlines with two experiments. The first experiment shows that a high context, low structure interface helps crowdworkers perform faster, higher quality synthesis, while the second experiment shows that a tournament-style (parallelized) crowd workflow produces faster, higher quality, more diverse outlines than a linear (serial/iterative) workflow.