Online Social Networks (OSNs) have an infamous history of privacy and security issues. One approach to avoid the massive collection of sensitive data of all users at a central point is a decentralized architecture.An event invitation feature – allowing a user to create an event and invite other users who then can confirm their attendance – is part of the standard functionality of OSNs. We formalize security and privacy properties of such a feature like allowing different types of information related to the event (e.g., how many people are invited/attending, who is invited/attending) to be shared with different groups of users (e.g., only invited/attending users).Implementing this feature in a Privacy-Preserving Decentralized Online Decentralized Online is non-trivial because there is no fully trusted broker to guarantee fairness to all parties involved. We propose a secure decentralized protocol for implementing this feature, using tools such as storage location indirection, ciphertext inferences and a disclose-secret-if-committed mechanism, derived from standard cryptographic primitives.The results can be applied in the context of Privacy-Preserving DOSNs, but might also be useful in other domains that need mechanisms for cooperation and coordination, e.g., Collaborative Working Environment and the corresponding collaborative-specific tools, i.e., groupware, or Computer-Supported Collaborative Learning.