Oh I loved this game back in the day... so much fun, though I think we'll need plenty of people. 12-16 is definitely the sweet spot.
For the un-initiated, here's a breakdown of how the game plays. First, the classes.
Soldier: Your basic run and gun fighter. Can use heavy weapons (MG42, Flamethrower, Panzerfaust, Mortar) to fight off the enemy. MG42 is an accurate machine gun if you go prone and deploy it, Flamethrower is self explanatory, Mortar fires indirect explosives a long way but uses energy, Panzerfaust shots rockets that use energy. Leveling up gives you more ammo, faster recharge of energy, higher speed, and an additional MP40/Thompson.
Medic: Self explanatory. Medics can toss med kits out, revive dead players, and regen their own health. Leveling lets them do it much better and use adrenaline on themselves. Uses MP40/Thompson as primary weapon.
Covert-Ops: Spies with lots of tricks. Have unique weapons (Sten, FG42, Sniper Rifle), silencers, and can drop satchel bombs to blow up some stuff. They can also steal enemy uniforms, drop smoke grenades, and detect landmines with their binoculars. Sten is a silence SMG but can overheat, while the FG42 is a fast firing, inaccurate gun when fired from the hip, but is a decent rifle when the scope is used. Leveling up lets them backstab, have more ammo, aim better while sniping and use bombs/smoke more often.
Field OPs: Commanders of the battle. Can drop ammo for allies, drop smoke to trigger a powerful airstrike, or use their binoculars to call an inaccurate artillery strike. Uses MP40/Thompson. Leveling lets them use their abilities more effectively and detect disguised enemies.
Engineer: The most important class. Engineers can build stuff to complete the mission, blow up enemy constructions and walls with dynamite, defuse dynamite, and place and remove landmines. The can use ether an MP40/Thompson or a rifle that does better damage and can fire off rifle grenades. Leveling up gives you more grenades and rifle grenades, a flak jacket, and the ability to do their job better.
The general idea behind each level is that your ether attacking or defending a location. If your attacking, you need to complete objectives to advance your assualt (like repairing a bridge, escorting a tank, stealing gold) until you finally complete the job. If your defending, all you need to do is hold off the attackers for a set period of time.
Along the way on each map, you can capture spawn points and construct radio spawns to make the assault easier.You get XP for kills and using the tools of your job.
I think that's all you really need to know.