In wireless-powered cooperative communication networks with multiple energy harvesting relays, relay selection has been the solution to exploit the cooperative diversity in the literature. In this paper, we propose to employ network beamforming (NB) in such networks to improve the source-to-destination capacity and reliability. More specifically, we propose a NB scheme with jointly optimized power splitting (PS) factors. To obtain the global optimum efficiently, semidefinite relaxation is used to convert the optimization problem to a quasi-convex one. To further reduce the communication overhead, we also propose a sub-optimal NB scheme with distributed PS factor optimization that requires only local channel state information. The performance gains of our proposed schemes compared with relay selection are demonstrated by simulations.