Reconfigurable intelligent surface (RIS) is considered one of the most promising technologies for beyond 5G and 6G communications with its potential to redefine the propagation environment and the simplicity of implementation. This paper proposes a closed-form low-complexity scheme for RIS configuration in a distributed multi-user MIMO (MU-MIMO) network and introduces a joint initial access and grouping scheme for RIS assignment. By leveraging the eigenvalue distribution of the access point-side channel, the proposed method aligns signals at orthogonal subspaces and effectively increases the degree of freedom for interference suppression. To verify the performance, we derive two achievable spectral efficiency (SE) equations for centralised and distributed operations. Numerical results show that the proposed scheme outperforms the baseline obviously under three different uplink combining methods.