Connected and Automated Vehicles (CAVs) have potential to improve the traffic efficiency near the highway on-ramps by sharing their merging times through wireless communication. For the traffic efficiency and vehicle safety, it has been the technical challenge to determine the merging times without collision of vehicles merging from different roads. Especially in the mixed traffic of CAVs and human-driven vehicles (hereinafter called non-CAVs), CAVs cannot detect all the non-CAVs even by their sensors. Therefore, it is necessary to determine CAVs' merging times for preventing the undetected non-CAVs from colliding with other vehicles. In this paper, we propose a group-based CAVs control method considering undetected non-CAVs in mixed traffic. The main idea of the proposed method is that it indirectly controls the undetected non-CAVs by CAVs. Specifically, the method forms vehicle groups each of which consists of a leading CAV and following non-CAVs. The method assigns merging times to the leading CAVs such that different groups do not merge onto the main road simultaneously. As a result, the undetected non-CAVs in a group do not collide with vehicles in the other groups. Simulation results show that the proposed method mitigates stop-and-go driving near the merging zone and improves outflow traffic.