  We combine optical and X-ray data for eight low redshift ($z\sim 0.06$) poor
groups of galaxies from the {\it XI}({\it XMM/IMACS}) Groups Project to study
the AGN population in the group environment. Among $\sim 140$ group members, we
identify five AGN based on their optical emission lines. None of these
optically-selected AGN are detected by {\it XMM-Newton}. One additional AGN is
discovered in the {\it XMM-Newton} observations. This X-ray detected AGN, which
has no obvious AGN emission line signatures in its optical spectrum, is a
member of the only X-ray luminous group in our sample. The lack of a
significant population of X-ray bright, but optically dull AGN among less
dynamically evolved groups is in stark contrast to the large fraction of such
objects in rich clusters of galaxies (Martini et al. 2006). We suggest this
result can be explained by a physical scenario for AGN accretion evolution: AGN
activity is initially triggered by galaxy merging, leading to a high accretion
rate and an optically dominant phase (via thin disk accretion). As the
accretion rate drops in time, the AGN gradually enters an X-ray dominant
low-accretion phase (via a radiative inefficient accretion flow). In this
picture, optical- and X-ray-selected AGN are the same population of
supermassive black holes observed at different epochs. Within the context of
this scenario, the majority of AGN in poor groups are in the high-accretion
optically dominant phase, while the AGN population in rich clusters is mostly
in the low-accretion X-ray dominant phase.
