Recently, to store a large amount of data for the blockchain decentralized applications, off-blockchain data storage techniques interworking with blockchain have emerged. However, these off-blockchain data storage techniques essentially cause the issue that it is difficult to control the user authentication and the access rights for each user outside the associated blockchain. In this paper, we introduce an implementation technique for supporting group workspaces based on the Ethereum blockchain, which provides secure file management among group users. To store large size files in group workspaces, a system structure is also being proposed where the Ethereum blockchain can stably interwork with the associated off–blockchain storage. Also, based on the file access rights stored in the Ethereum blockchain, we propose a systematic access control process for files shared among group members in the off-blockchain storage. In addition, to show the validity of the proposed technique, we present an Ethereum decentralized application called Bspace for using group workspaces interworking with off-blockchain storage.