We investigate the use of text and audio student-to-student communication during online lectures to foster social interaction, collaborative learning, and engagement. A prototype webcasting system called GroupLearn is presented, which allows students in "study groups" to communicate using audio conferencing and text chat. This system was evaluated in a user study wherein students from an undergraduate computer science class viewed pre-recorded lectures in an experimentally-controlled setting, under three peer-to-peer communication conditions: no communication, text chat only, and text chat and audio conferencing simultaneously. Both text chat and audio conferencing positively impacted user experience measures such as enjoyment, engagement, and isolation, although most students preferred to use text chat alone since audio conferencing could lead to interference with the lecture. Student learning was not significantly impacted by peer-to-peer communication overall, although there is evidence that text and audio communication affect learning and recall differently.