We investigate the effect of groups on a bi-directional flow, focusing on self-organisation phenomena, and more specifically on the time needed for the occurrence of pedestrian lanes, their stability and their effect on the velocity-density relation, and the amount of physical contact in the crowd. We use a novel collision avoidance model considering the asymmetrical shape of the human body, and combine it to a mathematical model of group behaviour. The presence of groups results to have a significant effect on velocity and lane organisation, and a dramatic one on collision. Despite the limitations of our approach, we believe that our results show the great theoretical and practical relevance of group behaviour in pedestrian models, and suggest that realistic results may hardly be achieved simply by adding together modular models.