  We report spectroscopic results from our 40-orbit $Hubble~Space~Telescope$
slitless grism spectroscopy program observing the 20 densest CARLA candidate
galaxy clusters at $1.4 < z < 2.8$. These candidate rich structures, among the
richest and most distant known, were identified on the basis of $[3.6]-[4.5]$
color from a $408~$hr multi-cycle $Spitzer$ program targeting $420$ distant
radio-loud AGN. We report the spectroscopic confirmation of $16$ distant
structures at $1.4 < z < 2.8$ associated with the targeted powerful
high-redshift radio-loud AGN. We also report the serendipitous discovery and
spectroscopic confirmation of seven additional structures at $0.87 < z < 2.12$
not associated with the targeted radio-loud AGN. We find that $10^{10} -
10^{11}\,M_{\odot}$ member galaxies of our confirmed CARLA structures form
significantly fewer stars than their field counterparts at all redshifts within
$1.4\leq z\leq 2$. We also observe higher star-forming activity in the
structure cores up to $z = 2$, finding similar trends as cluster surveys at
slightly lower redshifts ($1.0 < z < 1.5$). By design, our efficient strategy
of obtaining just two grism orbits per field only obtains spectroscopic
confirmation of emission-line galaxies. Deeper spectroscopy will be required to
study the population of evolved, massive galaxies in these (forming) clusters.
Lacking multi-band coverage of the fields, we adopt a very conservative
approach of calling all confirmations "structures", although we note that a
number of features are consistent with some of them being bona fide galaxy
clusters. Together, this survey represents a unique and large homogenous sample
of spectroscopically confirmed structures at high redshifts, potentially more
than doubling the census of confirmed, massive clusters at $z > 1.4$.
