  The RCS 2319+00 supercluster is a massive supercluster at z=0.9 comprising
three optically selected, spectroscopically confirmed clusters separated by <3
Mpc on the plane of the sky. This supercluster is one of a few known examples
of the progenitors of present-day massive clusters (10^{15} Msun by z~0.5). We
present an extensive spectroscopic campaign carried out on the supercluster
field resulting, in conjunction with previously published data, in 1961 high
confidence galaxy redshifts. We find 302 structure members spanning three
distinct redshift walls separated from one another by ~65 Mpc. The component
clusters have spectroscopic redshifts of 0.901, 0.905 and 0.905. The velocity
dispersions are consistent with those predicted from X-ray data, giving
estimated cluster masses of ~10^{14.5} - 10^{14.9} Msun. The Dressler-Shectman
test finds evidence of substructure in the supercluster field and a
friends-of-friends analysis identified 5 groups in the supercluster, including
a filamentary structure stretching between two cluster cores previously
identified in the infrared by Coppin et al. (2012). The galaxy colors further
show this filamentary structure to be a unique region of activity within the
supercluster, comprised mainly of blue galaxies compared to the ~43-77%
red-sequence galaxies present in the other groups and cluster cores. Richness
estimates from stacked luminosity function fits results in average group mass
estimates consistent with ~10^{13} Msun halos. Currently, 22% of our confirmed
members reside in >~10^{13} Msun groups/clusters destined to merge onto the
most massive cluster, in agreement with the massive halo galaxy fractions
important in cluster galaxy pre-processing in N-body simulation merger tree
studies.
