  We present the first mid-IR study of galaxy groups in the nearby Universe
based on Spitzer MIPS observations of a sample of nine redshift-selected groups
from the XMM-IMACS (XI) project, at z=0.06. We find that on average the
star-forming (SF) galaxy fraction in the groups is about 30% lower than the
value in the field and 30% higher than in clusters. The SF fractions do not
show any systematic dependence on group velocity dispersion, total stellar
mass, or the presence of an X-ray emitting intragroup medium, but a weak
anti-correlation is seen between SF fraction and projected galaxy density.
However, even in the densest regions, the SF fraction in groups is still higher
than that in cluster outskirts, suggesting that preprocessing of galaxies in
group environments is not sufficient to explain the much lower SF fraction in
clusters. The typical specific star formation rates (SFR/M*) of SF galaxies in
groups are similar to those in the field across a wide range of stellar mass
(M*>10^9.6 msun), favoring a quickly acting mechanism that suppresses star
formation to explain the overall smaller fraction of SF galaxies in groups. If
galaxy-galaxy interactions are responsible, then the extremely low starburst
galaxy fraction (<1%) implies a short timescale (~0.1 Gyr) for any
merger-induced starburst stage. Comparison to two rich clusters shows that
clusters contain a population of massive SF galaxies with very low SFR (14% of
all the galaxies with M*>10^10 Msun), possibly as a consequence of ram pressure
stripping being less efficient in removing gas from more massive galaxies.
