The integrated sensing and communications (ISAC) design has attracted great attention in recent years. Existing studies mainly focus on the monostatic radar sensing based on the assumption of full-duplex radio. However, since current legacy devices are all half-duplex radios, it is still important to investigate bistatic radar sensing based on half-duplex radios. In this paper, the 5G mmWave communication system with a hybrid beamforming architecture is considered, and the joint transmit and receive beamforming is designed for incorporating bistatic radar sensing function into the multi-user multi-input multi-output (MU-MIMO) communications. To overcome the critical problem of carrier-frequency-offset (CFO) and sampling-time-offset (STO) in bistatic sensing, the line-of-sight (LoS) signal is regarded as a virtual target for eliminating the CFO and STO. Therefore, the transmitter and radar receiver beamformers are jointly designed to maximize the radar signal-to-interference-plus-noise ratio (SINR), while satisfying the SINR constraints of both communication users and the virtual radar target. A hierarchical solution algorithm is developed to resolve this problem, in which a combined fully digital beamformer is first optimized for transmitter/sensing-receiver, and then the analog and digital beamformers for hybrid beamforming are derived from the fully digital beamformer. Simulation results show that compared to the separate beamforming design and the passive opportunistic sensing design, our joint design can achieve more than 5 dB and 7-22 dB radar sensing gain, respectively.