A large body of research has established the value of learner characteristics on cognitive load. However, little attention has been paid to the physical environment where learning takes place. The present study takes a step to address this gap by studying the impact of the presence of others during learning on cognitive load. In a between-subject design, participants (N = 115) were randomly arranged in groups of different group sizes to study computer-based multimedia materials (group size range: 1-13, continuous variable). Further, participants' working memory capacity, topic interest, and their prior knowledge were measured to reveal relevant learner characteristics. Dependent variables were learning performance, perceived task difficulty (mental load), and invested mental effort. We tested the predictions from cognitive load theory with alternative path models to identify the best model fit. Our results show that group size predicted learners' perceived task difficulty: the larger the group of co-actors in the learning situation was, the higher the perceived task difficulty. Moreover, higher topic interest led to lower perceived task difficulty, and more mental effort, although that effect became non-significant after multiple testing adjustment. Perceived task difficulty mediated the effect of group size and topic interest on mental effort.