Assuming by "format" you refer to composition (how many tanks, how many healers, how many rdps, how many mdps), that's a pretty simple answer: 2 tanks, 2 healers, and 4 dps (preferably 2 mdps and 2 rdps, with a preference for weighting towards more rdps if absolutely required). if by format you refer to how you should set up your groups, if you get a sentinel/marauder, you want to have tanks and healers in 1 group and dps in the other so that all of the DPS benefit from the Sent/Mara DPS buff(s) (+15% damage/healing is best applied to DPS rather than tanks or heals, since threat isn't a major concern for most skilled tanks and healers are better served by controlling their own burst healing).
As to what classes are best for what roles, it's important to note that the player behind a class is more important than the class itself. Even though a Scrapper Scoundrel is, by general consensus, the worst DPS class/spec out there, a well played one with be substantially more valuable than an idiot at the helm of an assault VG (I'm gonna be using the Republic classes and specs from here on out rather than the Imperial mirrors; if you're Imperial and not sure which is which, just ask and it'll be clarified).
For tanks, you're best served by getting 2 different tanks so that you have some play with picking the best tank class to deal with a specific mechanic. VG tanks are, functionally, the simplest tanks but their performance is pretty stagnant: a well played VG is going to perform about the same as a decently played one. A VG is a VG is a VG. Shadows have the absolute best top tier performance of the tanks, capable of doing some truly amazing feats of tanking when a really good player is at the helm. Guardians are hard to learn but the payoff is decent; they're roughly in the middle of VGs and Shadows when at the absolute top tier. As such, my recommendation, assuming you're picking up amazing players, is to grab a Shadow and a Guardian.
For healers, the same applies to healers as it does to tanks: you want to get 2 different classes. A Sage is a definite must since they are the, de facto, best AoE healers in the game. I would recommend a Scoundrel as your other healer since they're great at ST heals and are highly mobile (something Sages aren't; in essence, a Scoundrel healer covers the weaknesses of a Sage healer really well. Commandos are decent but, honestly, I think they leave something to be desired, going exclusively off of class capabilities: they're good ST healers, the worst AoE healers, and they're only really mobile when doing maintenance healing.
As to DPS, pretty much the only class/spec I'll counsel you against is Scrapper Scoundrel: their DPS leaves something to be desired and they've got some mobility/gap closing problems as a melee class (there are lots of knockbacks and forced movement periods for melee that don't really affect ranged). You definitely want to get at least 1 Sentinel for Inspiration (the previously mentioned 15% damage buff), with more being even better (the buff doesn't apply a debuff so, if you teach your Sents to stagger activation, you can benefit from each one). If you don't get a Guardian tank, you're going to want to get a Gunnery Commando, Gunslinger (of any spec), or Guardian DPS for the armor debuff. Going without it is going to really hurt your DPS. All of the other classes and specs, when built, played, and equipped properly, are going to do all the damage they need to do. Some people will counsel against Sages, Shadows, or Guardians, but the only reason for that is that those classes generally require a bit more player skill to eke out the same level of DPS that the other classes manage.
Great information!!!!! Listen to this guy, and I, having two of the normally advised against classes for dps (Sorcerer, and Juggernaut) or for Republic, (Sage, and Guardian) definitely recommend this for your dps section (if possible) Sentinel, Commando,Gunslinger, and Guardian.
Sentinel: Pretty good dps, and they have inspiration (15% damage increase)
Guardian: Good dps, armor debuff on bosses, can quickly switch to tank form if one of your tanks die and you need some time for the healers to get the fallen tank up
Commando: Good dps, armor debuff, can off-heal in bad situations
Gunslinger: Can pull very high dps, have a bubble that protects anyone in it from 20% of damage, very good in hairy situations.
This setup allows you to have two Melee DPS and two Ranged DPS, an off-tank, and off-healer, damage increase, and an oh-****! button. Also recommend Shadow tank and Guardian tank.