You'd need a phono preamp - this is usually built into a dj mixer.
You will need one for each turntable, and these are powered devices so you'll need a total of 5 electrical outlets.
The turntables (phono signal) will plug into the preamps, and a line-level signal will come out and go into the OT.
It's not only about grounding and amplification, the turntables by default have a very unique eq curve that the premp corrects for (its called RIAA eq curve if you want to learn more).
cool idea.