We report on a pilot implementation of a 'social and peer-to-peer' approach to Massive Online Open Courses (MOOCs) to scale participation in citizen science (CS) while maintaining scientific rigour. The GROW Observatory (GROW, EC H2020) is a citizens' observatory undertaking to deliver local and global impact by meaningfully engaging and supporting many people to take part in rigorous CS across wide geographic areas. MOOCs can deliver training to tens of thousands of people concurrently across a wide range of subjects as well as engender social and peer-to-peer – or ‘MOOC 2.0’ – models to promote social learning. We suggest these approaches may overcome common barriers to engagement, training for scientific protocols, data collection and sustainability currently experienced when supporting CS at scale. In GROW, a pilot MOOC was delivered to engage and support participants in the use of sensors and other equipment; site selection; methods design; data collection and data awareness. We present an analysis of our approach, and recommendations for future work.