What fraction of gravitational lens galaxies Lie in groups?

We predict how the observed variations in galaxy populations with environme nt affect the number and properties of gravitational lenses in different en vironments. Two trends dominate: lensing strongly favors early-type galaxie s, which tend to lie in dense environments, but dense environments tend to have a larger ratio of dwarf to giant galaxies than the field. The two effe cts nearly cancel, and the distribution of environments for lens and nonlen s galaxies are not substantially different (lens galaxies are slightly less likely than nonlens galaxies to lie in groups and clusters). We predict th at similar to 20% of lens galaxies are in bound groups (defined as systems with a line-of-sight velocity dispersion sigma in the range 200 < <sigma> < 500 km s(-1)), and another <similar to>3% are in rich clusters (sigma > 50 0 km s(-1)). Therefore, at least similar to 25% of lenses are likely to hav e environments that significantly perturb the lensing potential. If such pe rturbations do not significantly increase the image separation, we predict that lenses in groups have a mean image separation that is similar to0."2 s maller than that for lenses in the field, and we estimate that 20-40 lenses in groups are required to test this prediction with significance. The tail of the distribution of image separations is already illuminating. Although lensing by galactic potential wells should rarely produce lenses with imag e separations theta greater than or similar to 6 ", two such lenses are see n among 49 known lenses, suggesting that environmental perturbations of the lensing potential can be significant. Further comparison of theory and dat a will offer a direct probe of the dark halos of galaxies and groups and re veal the extent to which they affect lensing estimates of cosmological para meters.