  We have measured for the first time the weak lensing signal due to groups of
galaxies. The groups are at intermediate redshifts, and have been identified in
the CNOC2 field survey, which was kindly made available by Ray Carlberg and
Howard Yee. The ensemble averaged group velocity dispersion, based on a
preliminary selection of 59 groups, is found to be sigma= 320^{+46}_{-54} km/s,
which is in fair agreement with the dynamical estimate from the spectroscopic
redshifts. Under the assumption that mass traces light, we find an average
mass-to-light ratio in restframe B of (256\pm84)h Msun/Lsun. Using this result
we obtain Omega_m=0.22+-0.09 (\Omega_\Lambda=0).
