  Galaxy clusters are expected to form hierarchically in a LCDM universe,
growing primarily through mergers with lower mass clusters and the continual
accretion of group-mass halos. Galaxy clusters assemble late, doubling their
masses since z~0.5, and so the outer regions of clusters should be replete with
infalling group-mass systems. We present an XMM-Newton survey to search for
X-ray groups in the infall regions of 23 massive galaxy clusters at z~0.2,
identifying 39 X-ray groups that have been spectroscopically confirmed to lie
at the cluster redshift. These groups have mass estimates in the range
2x10^13-7x10^14Msun, and group-to-cluster mass ratios as low as 0.02. The
comoving number density of X-ray groups in the infall regions is ~25x higher
than that seen for isolated X-ray groups from the XXL survey. The average mass
per cluster contained within these X-ray groups is 2.2x10^14Msun, or 19% of the
mass within the primary cluster itself. We estimate that ~10^15Msun clusters
increase their masses by 16% between z=0.223 and the present day due to the
accretion of groups with M200>10^13.2Msun. This represents about half of the
expected mass growth rate of clusters at these late epochs. The other half is
likely to come from smooth accretion of matter not bound in halos. The mass
function of the infalling X-ray groups appears significantly top-heavy with
respect to that of field X-ray systems, consistent with expectations from
numerical simulations, and the basic consequences of collapsed massive dark
matter halos being biased tracers of the underlying large-scale density
distribution.
