Recently, offset-QAM based coherent WDM (CoWDM) has been proposed to build up spectrally-efficient multi-carrier superchannels. Compared with Nyquist wavelength division multiplexing (N-WDM) and orthogonal frequency division multiplexing (OFDM), offset-QAM based CoWDM can relax the stringent transmitter-side requirements for spectrum shaping and achieve significant transmission performance improvement. In order to efficiently utilize the sampling rate of commercially available analog-to-digital converter (ADC) and decrease the receiver-side implementation complexity, multi-carrier group detection scheme is investigated in offset-QAM based CoWDM where multiple carriers are simultaneously detected within single coherent receiver, followed by carrier separation in the digital domain through the 4-point discrete Fourier transform (DFT) method at the baseband. Here, we demonstrate a transmission of five-carrier 100 Gb/s polarization-multiplexed offset-16QAM signal with 12.5 GHz channel spacing. Through 3-carrier group detection, the sampling rate per-carrier is reduced to 1.33 times symbol rate in terms of 50 GS/s ADC and there is only 0.35 dB required OSNR penalty at BER=10-3 compared with conventional single channel coherent detection. Meanwhile, good tolerance of coherent receiver analog bandwidth is secured and receiver bandwidth is reduced to 8 GHz. Moreover, 0.5 dB required OSNR penalty at BER=10-3 is obtained given 18 GHz ADC bandwidth. Besides, we find that side carriers suffer from severer performance degradation than the central carrier with limited ADC resolution and only 0.08 dB and 0.2 dB required OSNR penalty at BER=10-3 are secured with 6 bits ADC resolution for central carrier and side carriers, respectively.