Let K be a totally real multi-quadratic fields and let $\mathcal{O}$ be its ring of integers. I would like to compute the orders of the finite subgroups of the discrete group $\mathrm{SL}_{2}(\mathcal{O})$. These orders are helpful for computing Euler characteristics.
I would like to know whether there is any methods to do the computations and where to find them. Thanks for all.