Uncle Pug:HID lights and passive communication.
A light, certainly. It doesn't need to be HID to communicate, ordinary halogen lights work fine (except for 38° or 43° beams on daylight dives, or LEDs).
Three person teams usually work fine in my experience. Best deployment I've been able to come up with is a wedge formation. Leader navigates and sets the pace and the two wingmen stay just a bit higher than the team leader, so they have a visual reference to each other. Communication is by touch for emergencies or interesting things to see, the dive leader usually checks the wingmen's position by watching the light beams on the bottom. The followers are supposed to check on each other while the team leader is navigating and should warn the dive leader by touch if there is an emergency. For routine "OK? OK!" checks, we use lights. For air checks, the leader signals a stop and turns do do the check.
When we stop for something interesting, positions may change, as long as there is visual reference. When we move on, we regroup first, then continue our way.
In my experience, it works better than a group of four. Three is a maximum IMO. Four is just two buddy teams.