  We present the results of a gravitational lensing analysis of the bright
$\zs=2.957$ sub-millimeter galaxy (SMG), HERMES J105751.1+573027 found in {\it
Herschel}/SPIRE Science Demonstration Phase data from the Herschel Multi-tiered
Extragalactic Survey (HerMES) project. The high resolution imaging available in
optical and Near-IR channels, along with CO emission obtained with the Plateau
de Bure Interferometer, allow us to precisely estimate the intrinsic source
extension and hence estimate the total lensing magnification to be $\mu=10.9\pm
0.7$. We measure the half-light radius $R_{\rm eff}$ of the source in the
rest-frame Near-UV and $V$ bands that characterize the unobscured light coming
from stars and find $R_{\rm eff,*}= [2.0 \pm 0.1]$ kpc, in good agreement with
recent studies on the Submillimeter Galaxy population. This lens model is also
used to estimate the size of the gas distribution ($R_{\rm eff,gas}=
[1.1\pm0.5]$) kpc by mapping back in the source plane the CO (J=5-4) transition
line emission. The lens modeling yields a relatively large Einstein radius
$R_{\rm Ein}= 4\farcs10 \pm 0\farcs02$, corresponding to a deflector velocity
dispersion of [$483\pm 16] \,\kms$. This shows that HERMES J105751.1+573027 is
lensed by a {\it galaxy group-size} dark matter halo at redshift $\zl\sim 0.6$.
The projected dark matter contribution largely dominates the mass budget within
the Einstein radius with $f_{\rm dm}(<R_{\rm Ein})\sim 80%$. This fraction
reduces to $f_{\rm dm}(<R_{\rm eff,G1}\simeq 4.5\kpc)\sim 47%$ within the
effective radius of the main deflecting galaxy of stellar mass $M_{\rm
*,G1}=[8.5\pm 1.6] \times 10^{11}\msun$. At this smaller scale the dark matter
fraction is consistent with results already found for massive lensing
ellipticals at $z\sim0.2$ from the SLACS survey.
