Finite groups of matrices over group rings
Gerald Cliff and Alfred Weiss. 
Trans. Amer. Math. Soc. 352 (2000), 457-475.

Abstract: We investigate certain finite subgroups $\Gamma$ of 
$GL_n(\mathbf Z\Pi)$, where $\Pi$ is a finite nilpotent group. Such a group 
$\Gamma$ gives rise to a $\mathbf Z[\Gamma\times\Pi]$-module; we study the 
characters of these modules to limit the structure of $\Gamma$. We also 
exhibit some exotic subgroups $\Gamma$.