Convergence Behaviour Archetypes (CBA) describe the many different ways that individuals spontaneously and collectively move towards an emergency situation. If this movement is not managed effectively, crisis management issues and problems can emerge and lead to an exacerbation of the crisis situation e.g. panic, convergence of people and resources towards danger, convergence of excess and unrequired people and resources etc. Users of social media platforms express different motivations and behaviours while converging on a crisis. While this behaviour has been analysed in previous research, an understanding of convergence behaviour facilitated by social media platforms to an effective level of control, is yet to be achieved. This paper examines how Twitter users, converged on the Manchester Bombing 2017. We identified the most impactful convergence behaviour archetypes, including those with the highest perceived legitimacy of convergence i.e. those deemed by the Twitter network, to have a necessary and meaningful role in the crisis. Manual content and social network analyses were conducted on our data by identifying three roles that determine the Twitter users with the highest impact regarding their retweet behaviour. We determined that Helpers, Mourners and Detectives had the highest impact on crisis communication in this event.