The anti-vaccination movement has successfully spread its views on social media. This study examined how community building emerges in the messages of Dutch anti-vaccination Telegram groups. Particularly, we investigated the extent to which these groups provide a platform for sharing information, perpetuating a shared identity, and promoting action. As negative emotions are considered a prime driver of collective action, we examined to what extent the messages had a negative valence. We used a mixed-method approach through a quantitative content analysis of 4654 text messages from five Telegram groups, while also examining the nature of the content through a qualitative analysis. The results suggest that most messages contained a form of shared identity (ingroup favoritism and outgroup hostility) or (mis)information, and, to a lesser extent, encouragements for (online) action. Moreover, most content had a negative valence. These findings illustrate how online groups might be sources of (mis)information, polarization, and intergroup hostility.