The GEEC2 spectroscopic survey of Galaxy Groups at $0.8<z<1$
  We present the data release of the Gemini-South GMOS spectroscopy in the
fields of 11 galaxy groups at $0.8<z<1$, within the COSMOS field. This forms
the basis of the Galaxy Environment Evolution Collaboration 2 (GEEC2) project
to study galaxy evolution in haloes with $M\sim 10^{13}M_\odot$ across cosmic
time. The final sample includes $162$ spectroscopically--confirmed members with
$R<24.75$, and is $>50$ per cent complete for galaxies within the virial
radius, and with stellar mass $M_{\rm star}>10^{10.3}M_\odot$. Including
galaxies with photometric redshifts we have an effective sample size of $\sim
400$ galaxies within the virial radii of these groups. We present group
velocity dispersions, dynamical and stellar masses. Combining with the GCLASS
sample of more massive clusters at the same redshift we find the total stellar
mass is strongly correlated with the dynamical mass, with
$\log{M_{200}}=1.20\left(\log{M_{\rm star}}-12\right)+14.07$. This stellar
fraction of $~\sim 1$ per cent is lower than predicted by some halo occupation
distribution models, though the weak dependence on halo mass is in good
agreement. Most groups have an easily identifiable most massive galaxy (MMG)
near the centre of the galaxy distribution, and we present the spectroscopic
properties and surface brightness fits to these galaxies. The total stellar
mass distribution in the groups, excluding the MMG, compares well with an NFW
profile with concentration $4$, for galaxies beyond $\sim 0.2R_{200}$. This is
more concentrated than the number density distribution, demonstrating that
there is some mass segregation.
