A fully-digital wideband joint communication-radar (JCR) with a multiple-input-multiple-output (MIMO) architecture at the millimeter-wave (mmWave) band will enable high joint communication and radar performance with enhanced design flexibility. A quantized receiver with few-bit analog-to-digital converters (ADCs) will enable a practical JCR solution with reduced power consumption for futuristic portable devices and autonomous vehicles. In this paper, we present a joint communication-radar proof-of-concept platform, named JCR70, to evaluate and demonstrate the performance of these JCR systems using real channel measurements in the 71-76 GHz band. We develop this platform by extending a mmWave communication set-up with an additional full-duplex radar receiver and by capturing the MIMO JCR channel using a moving antenna on a sliding rail. To characterize the JCR performance of our developed tested, we conduct several indoor and outdoor experiments and apply traditional as well as advanced processing algorithms on the measured data. Additionally, we compare the performance of our JCR70 platform with the INRAS Radarbook, which is a state-of-the-art automotive radar evaluation platform at 77 GHz. The results demonstrate that a quantized receiver with 2-4 bit ADCs generally performed quite close to the high-resolution ADC for a signal-to-noise ratio of up to 5 dB. Our JCR70 platform with a fully digital JCR waveform at 73 GHz and 2 GHz bandwidth achieved higher resolution capability than the Radarbook due to higher bandwidth and larger synthesized antenna aperture.