In this study, we focus on participants called lurkers, who do not post any messages in an online community such as interactive mailing lists and bulletin board systems. We propose a method of classifying participants including lurkers based on two criteria: what types of actions they take outside the online community, and whether or not the online community affects their thoughts. In addition, based on the results of interviews, we propose hypotheses regarding factors that characterize the categories of lurkers. We conduct a questionnaire survey of all participants in two in-house online communities to verify our method and test the hypotheses. There are a considerable number of lurkers who have a strong and wide influence outside the online community. We conclude that such lurkers cannot be neglected in an evaluation of online communities within a company. We also discuss the possibility of online community management by focusing on not only posters but also lurkers who are indirect contributors to increasing the influence of an online community on its outside environment.