This paper presents a low-complexity adaptive scheme for an uplink multiuser transmission in a massive multiple-input and multiple-output (MIMO) system using maximum-ratio combining (MRC) at the receiver. Instead of using small-scale fast-varying fading, slow-varying large-scale shadowing information is exploited to adapt the desired mode among a discrete set of modulations available at the transmitter side. Since users experience different large-scale fading parameters, the adaptation procedure has to be calculated separately for each user. Therefore, this scheme is called multiuser adaptive modulation (MAM) and is compared to the traditional approach called fast adaptive modulation (FAM), which accounts for small-scale fast-varying fading for adaptation. Furthermore, the closed-form expressions for the average spectral efficiency and bit error outage are derived. In comparison with the FAM scheme and the previous research results, the outcome of this paper shows that at low transmit power, the aforementioned adaptive scheme by the MRC receiver gives a similar performance.