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We present a weak gravitational lensing analysis of supergroup SG1120$-$1202,consisting of four distinct X-ray-luminous groups, that will merge to form acluster comparable in mass to Coma at $z=0$. These groups lie within aprojected separation of 1 to 4 Mpc and within $\Delta v=550$ km s$^{-1}$ andform a unique protocluster to study the matter distribution in a coalescingsystem.  Using high-resolution {\em HST}/ACS imaging, combined with an extensivespectroscopic and imaging data set, we study the weak gravitational distortionof background galaxy images by the matter distribution in the supergroup. Wecompare the reconstructed projected density field with the distribution ofgalaxies and hot X-ray emitting gas in the system and derive halo parametersfor the individual density peaks.  We show that the projected mass distribution closely follows the locations ofthe X-ray peaks and associated brightest group galaxies. One of the groups thatlies at slightly lower redshift ($z\approx 0.35$) than the other three groups($z\approx 0.37$) is X-ray luminous, but is barely detected in thegravitational lensing signal. The other three groups show a significantdetection (up to $5 \sigma$ in mass), with velocity dispersions between$355^{+55}_{-70}$ and $530^{+45}_{-55}$ km s$^{-1}$ and masses between$0.8^{+0.4}_{-0.3} \times 10^{14}$ and $1.6^{+0.5}_{-0.4}\times 10^{14} h^{-1}M_{\odot}$, consistent with independent measurements. These groups areassociated with peaks in the galaxy and gas density in a relativelystraightforward manner. Since the groups show no visible signs of interaction,this supports the picture that we are catching the groups before they mergeinto a cluster.

The mass distribution in an assembling super galaxy group at $z=0.37$