Citizen Inquiry has been proposed as an informal science learning approach to enable widespread involvement in science and empower citizens with reasoning and problem-solving skills used by scientists. It combines aspects from citizen science and inquiry-based learning, producing science learning experiences within distributed communities of interest. A central challenge for Citizen Inquiry is to involve citizens in planning and implementing their own investigations, supported and guided by online systems and tools within an inquiry environment, while collaborating with science experts and non-experts. This thesis explores how to create an active and sustainable online community for citizens to engage in scientific investigations. To this end, it investigates the design of online communities, recruitment and retaining of members, factors that engage or disengage members from the community, and whether and how members learn throughout their participation. The intervention comprises two iterations of Citizen Inquiry communities: ‘Inquiring Rock Hunters’ and ‘Weather-it’. The communities were accommodated by the nQuire platform and the nQuire-it toolkit, respectively, software designed and structured to support collaborative personally-meaningful inquiry learning. The findings of this research are explained through an analysis that compared the two design studies with previous research on citizen participation projects and online communities. Results highlight the importance of frequent project communication, multiple ways of participation, software usability, and interaction and collaboration between the members, while indicating disengagement factors such as lack of time, interest and confidence. Different categories of learning are identified (activity, on-topic and community), emphasizing the understanding of inquiry activities as part of a complete scientific process and the balance between fun and learning. The thesis concludes with design considerations for the creation of future Citizen Inquiry and other citizen participation communities.