Social movements facilitated by online communication platforms such as social media are known for their sudden appearance and rapid mobilization. But there has been a lack of studies examining the potential for social influence to mobilize support for activist organizations in the online realm. This study bridges the gap by examining individuals' willingness to participate in different forms of collective activities (e.g., online versus offline, low- versus mid- versus high thresholds) upon receiving invitational messages from different sources online through a communicative framework for collective action and "online connective action." An online between-groups, post-test-only experiment was carried out, manipulating for message sources (e.g., close personal circles, distant social network, experts and authority figures, organizational sources) and controlling for dispositional variables (e.g., personal involvement with activism and issue, ideological orientation, attitude toward activism, and perceived efficacy). Data was collected from 422 undergraduates enrolled in a large southeastern university in the United States. Findings revealed that messages passed by members in one's close personal networks are significantly more influential than those passed by less personal sources. Individuals also expressed greater willingness to participate in online forms of collective activities (e.g., provide donations, sign e-petitions, post comments, relaying the message, "Like" the message, "Follow" the online group, etc.) as compared to offline ones (e.g., participate in public demonstrations, distribute campaign materials, use campaign items in public, etc.). Interestingly, contrary to prior postulations, findings showed that individuals were more willing to participate in online activities that compromised certain levels of privacy as opposed to anonymous ones. An online source influence model for collective action was then tested and forwarded in this study. Discussions elaborated on the significance of the "personal" nature of influence for collective action and the diminishing role of the organization in the online realm; suggesting the possible proliferation of grassroots activism propelled by today's socially networked online environment. From an interpersonal communicatory effects viewpoint, the findings also provided evidence on the importance of a communicative approach in explaining online collective action beyond the discursive perspectives that are widely adopted by prior studies.