One growing use of computer-based communication media is for gathering people to initiate or sustain social events. Although the use of computer-mediated communication and social network sites such as Facebook for event promotion is becoming popular, online participation in an event does not always translate to offline attendance. In this paper, we report on an interview study of 31 participants that examines how people handle online event invitations and what influences their online and offline participation. The results show that people's event participation is shaped by their social perceptions of the event's nature (e.g., public or private), their relationships to others (e.g., the strength of their connections to other invitees), and the medium used to communicate event information (e.g., targeted invitation via email or spam communication via Facebook event page). By exploring how people decide whether to participate online or offline, the results illuminate the sophisticated nature of the mechanisms that affect participation and have design implications that can bridge virtual and real attendance.