  Augmenting the \emph{WFIRST} microlensing campaigns with intensive
observations from a ground-based network of wide-field survey telescopes would
have several major advantages. First, it would enable full two-dimensional
(2-D) vector microlens parallax measurements for a substantial fraction of
low-mass lenses as well as planetary and binary events that show caustic
crossing features. For a significant fraction of the free-floating planet (FFP)
events and all caustic-crossing planetary/binary events, these 2-D parallax
measurements directly lead to complete solutions (mass, distance, transverse
velocity) of the lens object (or lens system). For even more events, the
complementary ground-based observations will yield 1-D parallax measurements.
Together with the 1-D parallaxes from \emph{WFIRST} alone, they can probe the
entire mass range $M\gtrsim M_\oplus$. For luminous lenses, such 1-D parallax
measurements can be promoted to complete solutions (mass, distance, transverse
velocity) by high-resolution imaging. This would provide crucial information
not only about the hosts of planets and other lenses, but also enable a much
more precise Galactic model. Other benefits of such a survey include improved
understanding of binaries (particularly with low mass primaries), and
sensitivity to distant ice-giant and gas-giant companions of \emph{WFIRST}
lenses that cannot be detected by \emph{WFIRST} itself due to its restricted
observing windows. Existing ground-based microlensing surveys can be employed
if \emph{WFIRST} is pointed at lower-extinction fields than is currently
envisaged. This would come at some cost to the event rate. Therefore the
benefits of improved characterization of lenses must be weighed against these
costs.
