Social media provides the means by which extremist social movements, such as white supremacy and anti-LGBTQ, thrive online. Yet, we know little about the roles played by the participants of such movements. In this paper, we investigate these participants to characterize their roles, their role dynamics, and their influence in spreading online extremism. Our participants-online extremist accounts-are 4,876 public Facebook pages or groups that have shared information from the websites of 289 Southern Poverty LawCenter (SPLC) designated extremist groups. Guided by theories of participatory activism, we map the information sharing features of these extremists accounts. By clustering the quantitative features followed by qualitative expert validation, we identify five roles surrounding extremist activism-educators, solicitors, flamers, motivators, sympathizers. For example, solicitors use links from extremist websites to attract donations and participation in extremist issues, whereas flamers share inflammatory extremist content inciting anger. We further investigate role dynamics such as, how stable these roles are over time and how likely will extremist accounts transition from one role into another. We find that roles core to the movement-educators and solicitors-are more stable, while flamers and motivators can transition to sympathizers with high probability. Finally, using a Hawkes process model, we test which roles are more influential in spreading various types of information. We find that educators and solicitors exert the most influence in triggering extremist link posts, whereas flamers are influential in triggering the spread of information from fake news sources. Our results help in situating various roles on the trajectory of deeper engagement into the extremist movements and understanding the potential effect of various counter-extremism interventions. Our findings have implications for understanding how online extremist movements flourish through participatory activism and how they gain a spectrum of allies for mobilizing extremism online.