  This paper is the first in a series in which we perform an extensive
comparison of various galaxy-based cluster mass estimation techniques that
utilise the positions, velocities and colours of galaxies. Our primary aim is
to test the performance of these cluster mass estimation techniques on a
diverse set of models that will increase in complexity. We begin by providing
participating methods with data from a simple model that delivers idealised
clusters, enabling us to quantify the underlying scatter intrinsic to these
mass estimation techniques. The mock catalogue is based on a Halo Occupation
Distribution (HOD) model that assumes spherical Navarro, Frenk and White (NFW)
haloes truncated at R_200, with no substructure nor colour segregation, and
with isotropic, isothermal Maxwellian velocities. We find that, above 10^14
M_solar, recovered cluster masses are correlated with the true underlying
cluster mass with an intrinsic scatter of typically a factor of two. Below
10^14 M_solar, the scatter rises as the number of member galaxies drops and
rapidly approaches an order of magnitude. We find that richness-based methods
deliver the lowest scatter, but it is not clear whether such accuracy may
simply be the result of using an over-simplistic model to populate the galaxies
in their haloes. Even when given the true cluster membership, large scatter is
observed for the majority non-richness-based approaches, suggesting that mass
reconstruction with a low number of dynamical tracers is inherently
problematic.
