The Signal protocol is a secure messaging protocol providing end-to-end encrypted asynchronous communication. In this paper, we focus on a method capable of hiding membership information from the viewpoint of non group members in a secure group messaging (SGM) protocol, which we call “membership privacy”. Although Chase et al. (ACM CCS 2020) have considered the same notion, their proposal is an extension of Signal so called “Pairwise Signal” where a group message is repeatedly sent over individual Signal channels. Thus, for the number of group users n, their protocol is not scalable where each user is required O(n) computational and communication costs for updating keys. In this work, we extend the Cohn-Gordon et al. SGM protocol (ACM CCS 2018), which we call the Asynchronous Ratcheting Trees (ART) protocol, to add membership privacy. The ART protocol is scalable where each user is required computational and communication costs for updating keys. We employ a key-private and robust public-key encryption (Abdalla et al., TCC2010/JoC2018) for hiding membership-related values in the setup phase. Furthermore, we concentrate on the fact that a group common key provides anonymity. This fact is used to encrypt membership information in the key update phase. Our extension does not affect the forward secrecy and post-compromise security of the original ART protocol. Our modification achieves asymptotically the same efficiency of the ART protocol in the setup phase. Any additional cost for key update does not depend on the number of group members (specifically, one encryption and decryption of a symmetric-key encryption scheme and one execution of a key-derivation function for each key update are employed). Therefore, the proposed protocol can add membership privacy to the ART protocol with a quite small overhead.