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 submillimetre galaxy (SMG) HDF850.1. We have selected candidate cluster members from the optical 25 medium-band photometric catalogue of the project SHARDS (Survey for High-z Absorption Red and Dead Sources). 17 rest-frame ultraviolet (UV)-selected galaxies (LAEs and LBGs) with 5.15 < zphot < 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 α line at the redshift of the galaxy protocluster PCl-HDF850.1, demonstrating the high reliability 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 data set with the sample from the literature. Beside the SMG HDF850.1, none of the 23 spectroscopically confirmed members are bright in the far-infrared/sub-mm wavelength regime (SFRIR < few hundred M⊙ 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 Δz < 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 and $8\times 10^{12}\, \mathrm{M}_{\odot }$ . 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 ( $\gt 10^{15}\, \mathrm{M}_{\odot }$ ).