  Clusters and their progenitors (protoclusters) at z = 2-4, the peak epoch of
star formation, are ideal laboratories to study the formation process of both
the clusters themselves and their member galaxies. However, a complete census
of their member galaxies has been challenging due to observational
difficulties. Here we present new JWST/NIRCam observations targeting the
distant cluster CLJ1001 at z = 2.51 from the COSMOS-Web program, which, in
combination with previous narrowband imaging targeting H-alpha emitters and
deep millimeter surveys of CO emitters, provide a complete view of massive
galaxy assembly in CLJ1001. In particular, JWST reveals a population of
massive, extremely red cluster members in the long-wavelength bands that were
invisible in previous Hubble Space Telescope (HST)/F160W imaging (HST-dark
members). Based on this highly complete spectroscopic sample of member
galaxies, we show that the spatial distribution of galaxies in CLJ1001 exhibits
a strong central concentration, with the central galaxy density already
resembling that of low-z clusters. Moreover, we reveal a "top-heavy" stellar
mass function for the star-forming galaxies (SFGs), with an overabundance of
massive SFGs piled up in the cluster core. These features strongly suggest that
CLJ1001 is caught in a rapid transition, with many of its massive SFGs likely
soon becoming quiescent. In the context of cluster formation, these findings
suggest that the earliest clusters form from the inside out and top to bottom,
with the massive galaxies in the core assembling first, followed by the less
massive ones in the outskirts.
