In this paper, we describe Mobile CoWPI, a deployable, end-to-end secure mobile group messaging application with proofs of security. Mobile CoWPI allows dynamic groups of users to participate in, join, and leave private, authenticated conversations without requiring the participants to be simultaneously online or maintain reliable network connectivity. We identify the limitations of mobile messaging and how they affect conversational integrity and deniability. We define strong models of these security properties, prove that Mobile CoWPI satisfies these properties, and argue that no protocol that satisfies these properties can be more scalable than Mobile CoWPI. We also describe an implementation of Mobile CoWPI and show through experiments that it is suitable for use in real-world messaging conditions.