  The effect of stars on the lensing properties of the supermassive black hole
in the Galactic Center is similar to the effect of planets on microlensing by a
star. We show that the dense stellar cluster around SgrA* increases by factors
of a few the probability of high-magnification lensing events of a distant
background source by the black hole. Conversely, the gravitational shear of the
black hole changes and enhances the microlensing properties of the individual
stars. The effect is largest when the source image lies near the Einstein
radius of the black hole 1.75"+/-0.20" for a source at infinity). We estimate
that the probability of observing at least one distant background star which is
magnified by a factor >5 in any infrared snapshot of the inner ~ 2" of the
Galactic Center is 1% with a K-band detection threshold of 20 mag. The largest
source of uncertainty in this estimate is the luminosity function of the
background stars. The gravitational shear of the black hole lengthens the
duration of high-magnification events near the Einstein radius up to a few
months, and introduces a large variety of lightcurve shapes that are different
from those of isolated microlenses. Identification of such events by image
subtraction can be used to probe the mass function, density and velocity
distributions of faint stars near the black hole, which are not detectable
otherwise.
