I'd say you have four options here:
The players share one tank, Comrades man a 2nd one. This has the advantage of keeping the players together in their little tin, and they can use their Comrades' vehicle as cover or to take the worst of a beating. After all, normally Comrades have a chance at "catching a bullet" for a PC, but with a tank this option does not exist in the same way. Giving the Comrades their own tank reintroduces this perk.
Each tank is manned by 2 players and 2 Comrades. This allows two of your players to act as drivers, and two to act as tank commanders. The Comrades are all Gunners and will man the turret and hull weapon.
Take a Leman Russ with sponsons (for a total of 6 crew), and just forget about the remaining 2 Comrades. Assign the two Comrades who still find room based on what their players are doing, meaning, in a way that each of your players has as much stuff to do as the next one. For example, you could have the driver and the hull gunner each gain a sponson gunner in addition to their primary job, whereas the tank commander and the turret gunner do not, because their position is already awesome enough (one gets to shout orders and tell everyone what to do, the other gets to fire the biggest cannon of 'em all).
The players crew a tank, Comrades are infantry who sit on the back. The NPCs will dismount upon combat and assist the vehicle.
(note that the last two options necessitate houseruling the type of Comrade a player class gets)