7.5 Rogues roll in a group
First and foremost a rogues roll in a group will almost always be to take out the healer/caster 1st. Therefore in PvE you should go for enemies in this order:
Caster Healer (Priest, Shaman and other healers) > Caster (Mage, Warlocks) > Ranged (Hunters) > Melee (Warriors, Brutes).
The reason is that your stuns/interupts (Gouge, Kick, KS, CS, Blind) are designed to keep a caster from casting and they usually have less armor so they will go down quickly with your DPS. Also, they are very powerful if allowed to cast. If you go for the warrior first the caster will be just plugging away at you or worse....healing the warrior to full negating all the damage you just did. =[
Rogues role in a duo:
If you are with a tanking class like a Warrior, Paladin or Druid (bear form) Your job is to effectively dish out DPS without pulling too much agro. Backstabs and feints are your friends. If you tank is getting hurt too much you CAN pull agro and tank with evasion for awhile. If both of you are getting hurt start using KS to buy some time. If you are grouped with a Healer you ARE the tank. Don't let the healer pull agro make him concentrate on laying on a few dots, nukes and healing you while you DPS like mad. If you need to you can always evasion or use your stuns. If you are with another Rogue or Ranged DPS class, just DPS them down. If your buddy ever needs any help you can easily pull the agro back and if you are getting too much feint or vanish.
Rogues in a 5-man to 10-man:
Sap and Improved Sap are great utilities for a rogue. Lockpicking will help too. Your main role will still be to DPS but try to focus on the one that your Tank is currently working on, that way you won't pull agro and can help him drop it down quicker. Try using a /assist macro. You will still want to focus on healers and casters mostly. A secondary function will be to Off-tank if one of your casters pulled agro. Your Tank might not notice one of the mobs running towards your priest but you did. Backstab him to get his attention and drag him back to the tank to regain agro.
Rogues in 10 man and up:
Your primary function, as always, is DPS. You are known as Melee and serve a vital role of keeping the encounter at casting range from your casters. Even though you will often be grouped with Tanks, MS/Fury Warriors, Paladins/Shaman, and possibly druids in Melee you need to watch your own health bar. The Healers are focused on keeping the tanks alive and other warriors as well, as their increased damage mitigation makes them more mana efficient targets for heals. Druids, Shaman and Pallys can heal themselves and you are required to as well. (as silly as that sounds) So, bring lots of bandages and health pots.