  We exploit the group environment of the CAmbridge Sloan Survey Of Wide ARcs
in the skY (CASSOWARY) z=0.3 lens J2158+0257 to measure the group dynamical
mass as a complement to the central dynamical and lensing mass constraints.
Follow-up spectroscopy of candidate group members is performed using VLT/FORS2.
From the resulting N=21 confirmed members we measure the group dynamical mass
by calibrating an analytic tracer mass estimator with cosmological simulations.
The luminosity weighted line-of-sight velocity dispersion and the Einstein
radius of the lens are used as mass probes in the inner regions of the galaxy.
Combining these three observational probes allows us to independently constrain
the mass and concentration of the dark matter halo, in addition to the total
stellar mass of the central galaxy. We find a dark matter halo in remarkably
good agreement with simulations (log_10 M_200/M_sun = 14.2 +/- 0.2, c_200 = 4.4
(+1.6, -1.4)) and a stellar mass-to-light ratio which favors a Salpeter initial
mass function ((M/L)* = 5.7 +/- 1.2). Our measurement of a normal halo
concentration suggests that there is no discrepancy between simulations and
observations at the group mass scale. This is in contrast to the cluster mass
scale for which a number of studies have claimed over-concentrated halos. While
the halo mass is robustly determined, and the halo concentration is not
significantly affected by systematics, the resulting stellar mass-to-light
ratio is sensitive to the choice of stellar parameters, such as density profile
and velocity anisotropy.
