  We present galaxy velocity dispersions and dynamical mass estimates for 44
galaxy clusters selected via the Sunyaev-Zel'dovich (SZ) effect by the Atacama
Cosmology Telescope. Dynamical masses for 18 clusters are reported here for the
first time. Using \Nbody\ simulations, we model the different observing
strategies used to measure the velocity dispersions and account for systematic
effects resulting from these strategies. We find that the galaxy velocity
distributions may be treated as isotropic, and that an aperture correction of
up to 7 per cent in the velocity dispersion is required if the spectroscopic
galaxy sample is sufficiently concentrated towards the cluster centre.
Accounting for the radial profile of the velocity dispersion in simulations
enables consistent dynamical mass estimates regardless of the observing
strategy. Cluster masses $M_{200}$ are in the range
$(1-15)\times10^{14}M_\odot$. Comparing with masses estimated from the SZ
distortion assuming a gas pressure profile derived from X-ray observations
gives a mean SZ-to-dynamical mass ratio of $1.10\pm0.13$, but there is an
additional 0.14 systematic uncertainty due to the unknown velocity bias; the
statistical uncertainty is dominated by the scatter in the mass-velocity
dispersion scaling relation. This ratio is consistent with previous
determinations at these mass scales.
