Online communities can serve as meaningful sources of social support, particularly for marginalized and vulnerable groups. Disclosure of personal information facilitates integration into online communities but may also expose individuals to harm, including cyberbullying and manipulation. To better understand negative user experiences resulting from self-disclosure in online conversations, we interviewed 25 participants from target populations on Reddit. Through thematic analysis, we outline the harm they experience, including damage to self- and group identities. We find that encountering online harm can worsen offline adversity. We discuss how users protect themselves and recover from harm in the context of current platform affordances, highlighting ongoing challenges. Finally, we explore design implications for a community-driven, bottom-up approach to enhance user well-being and safety.