  We present JWST/NIRSpec Integral Field Spectroscopy in the rest-frame optical
bands of the system PJ308-21, a quasar at $z=6.2342$ caught as its host galaxy
interacts with companion galaxies. We detect spatially extended emission of
several emission lines (H$\alpha$, H$\beta$, [OIII], [NII], [SII], HeII), which
we use to study the properties of the ionized phase of the interstellar medium:
the source and hardness of the photoionizing radiation field, metallicity, dust
reddening, electron density and temperature, and star formation. We also
marginally detect continuum starlight emission associated with the companion
sources. We find that at least two independent satellite galaxies are part of
the system. While the quasar host appears highly enriched and obscured, with
AGN-like photoionization conditions, the western companion shows minimal dust
extinction, low metallicity ($Z\sim0.4$ Z$_\odot$), and star-formation driven
photoionization. The eastern companion shows higher extinction and metallicity
($Z\sim0.8$ Z$_\odot$) compared to the western companion, and it is at least
partially photoionized by the nearby quasar. We do not find any indication of
AGN in the companion sources. Our study shows that while the quasar host galaxy
is already very massive ($M_{\rm dyn}>10^{11}$ M$_\odot$), it is still rapidly
building up by accreting two relatively massive ($M_{\rm star}\sim 10^{10}$
M$_\odot$) companion sources. This dataset showcases the power of JWST in
exposing the build-up of massive galaxies in the first Gyr of the Universe.
