This paper studies optimal distributed beamforming designs to jointly maximize the signal-to-noise (SNR) margin in a multiuser multi-relay network. Considered are optimization problems with two different types of power constraints: sum relay power constraint and per-relay power constraints. Although these two problems can be readily solved by the bisection method via a sequence of second-order conic feasibility programs, we propose simple and fast converging iterative algorithms to directly solve the two optimization problems under consideration.