The Atacama Cosmology Telescope Polarisation experiment (ACTPol) is building a redshift independent, approximately mass limited galaxy cluster sample by exploiting the Sunyaev-Zel'dovich (SZ) effect. The goal of this effort is to measure cosmological parameters by charting the growth of massive structures in the Universe. In this talk I will describe the construction of the cluster sample in the 900 square degree ACTPol D56 field, and the associated optical/IR follow-up program. I will also describe recent efforts to improve the calibration between SZ signal and mass, which is crucial for improving constraints on cosmological parameters, using ACT clusters with weak lensing and dynamical mass measurements.