The social identity approach to leadership argues that leaders’ capacity to influence and inspire others is grounded in a shared sense of social identity (or ‘us-ness’) that those leaders create, advance, represent, and embed for the groups they lead. The approach therefore argues that a key task for leaders is to develop insights and skills of (social) identity leadership that allow them to motivate and mobilize groups and transform them into a potent social and organizational force. In contrast to other approaches and programs which focus on leaders’ leader identity (their ‘I-ness’), the 5R leadership development program supports the development of leaders’ social identity by raising awareness of the importance of social identity (‘we-ness’) for leadership and taking leaders through structured activities that help them build engaged and inclusive teams. The present research assessed the benefits of facilitated and learner self-directed versions of the 5R program (Ns = 27, 22 respectively) relative to a no-treatment control (N= 27). Results (including those of an intention-to-treat analysis;N= 76) indicated that, relative to leaders in the control condition, those who participated in both forms of 5R reported large increases in identity leadership knowledge, as well as medium-sized increases in both team engagement (a compound factor comprised of team identification, team OCB, team efficacy, and work engagement) and ‘teamfulness’ (comprised of team reflexivity, team psychological safety, team goal clarity, and inclusive team climate). We reflect on the importance of teamfulness for leadership and team functioning and on the value of programs that help leaders develop this.