This paper presents MOSAIC (Multimodal Observations of Social Affect, Intimacy, and Cohesion), a multimodal interaction corpus of video and audio recordings of 17 groups of 4 participants (68 participants in total) playing a collaborative board game. The aim of this corpus collection is to support the design of socially interactive agents. We describe the experimental protocol of this corpus collection, the perceptive questionnaires completed by participants, the automatic annotation process of game specific elements and non-verbal behaviors, and the manual verbal annotations collected. We provide a preliminary characterization of this corpus with descriptive results for some of the perceptive scales used in this study. We illustrate the possibilities offered by this corpus on the question of interpersonal social relations and group dynamics.