This paper investigates the resource allocation problem for uplink multiple-access relay channels (MARC) with network coding. Firstly, the difference between traditional relay channels and MARC is presented, and then a joint optimization problem considering power allocation and relay selection is formulated to maximize the sum-rate under total power constraint. The optimal algorithm is proposed by optimizing two separate sub problems and then two low-complexity suboptimal algorithms are further given. The simulation results demonstrate performance gain of the proposed optimal algorithm over the traditional four time-slot scheme. Moreover, the impact of total transmission power and relay number on system performance is also evaluated.