  We report the results of a pilot spectroscopic program of a region at z=5.2
in the GOODS-N field containing an overdensity of galaxies around the
well-known submillimeter galaxy HDF850.1. We have selected candidate cluster
members from the optical 25 medium-band photometric catalog of the project
SHARDS (Survey for High-z Absorption Red and Dead Sources). 17 rest-frame UV
selected galaxies (LAEs and LBGs) with 5.15<z_phot<5.27, candidates to be
physically associated with the overdensity, have been observed with the
instrument OSIRIS at the GranTeCan telescope. 13 out of these 17 (76 per cent)
sources have secure spectroscopic confirmations via the Ly$\alpha$ line at the
redshift of the galaxy protocluster PCl$-$HDF850.1, demonstrating the high
reliabilty of our photometric redshift method. 10 out of 13 sources are newly
confirmed members. Thus, we increase the number of confirmed members in this
overdensity from 13 to 23 objects. In order to fully characterize this
structure we combined our dataset with the sample from the literature. Beside
the SMG HDF850.1, none of the 23 spectroscopically confirmed members are bright
in the far-infrared/submm wavelength regime (SFR_IR< few hundred M_sun yr^-1).
The clustering analysis of the whole sample of 23 confirmed members reveals
four distinct components in physical space in different evolutionary states,
within Delta_z<0.04 from the central region hosting SMG HDF850.1. The halo mass
of the whole structure at z=5.2, estimated by a variety of methods, range
between 2-8 x 10^12 M_sun. The comparison with literature suggests a large
scale assembly comparable to the formation of a central Virgo-like cluster at
z=0 with several satellite components which will possibly be incorporated in a
single halo if the protocluster is the progenitor of a more massive Coma-like
cluster (>10^15 M_sun).
