This study considers linear precoding for secure transmission in a multiuser peer-to-peer relay network with finite alphabet input. Under the assumption that the global channel-state-information is available, the achievable secrecy rate is derived. However, the computational complexity to evaluate the achievable secrecy rate grows exponentially with respect to the number of pair users. To reduce the computational complexity caused by the multiuser interference, an accurate approximation of the achievable secrecy rate is derived. Based on Karush-Kuhn-Tucker analysis, necessary conditions for the optimal precoder which maximises the approximated achievable secrecy rate are presented. In light of this, an iterative gradient method is developed to find the optimal precoder. Numerical examples demonstrate that the proposed scheme achieves significant gains in terms of the secrecy rate over schemes designed for Gaussian input.