Social software (SSW), nowadays increasingly widespread, has excellent potential for use as persuasive technology. What differentiates it from many other persuasive technology platforms is that it is inherently collective, making group dynamics a powerful factor in any SSW context of persuasion. Based on the psychology of groups, persuasion, and cross-cultural theory, we discuss affiliation, access, and participation as themes that are important in understanding SSW's use as a persuasive technology platform.