Similar to groups of PVE you'll be the one responsible for destroying particular targets with WoTLK Gold weapons, but you'll need to be more vigilant about surviving because you're not able to count on your warriors to keep the fight going. In contrast you are able to cast AE spells without attracting the attention of all enemies who is on the other side, and so spells such as Flamestrike or Blizzard are fantastic to cast in the area that the majority of your melee class members are battling out. If you stick with spells such as these, you likely will not draw attention to yourself. However, should you choose to cast Improved Arcane Explosion when standing in front of the melee players they'll make you an immediate target.
Alongside pure damage, you can also use your crowd control spells and rooting spells to wreak destruction to enemy troops. Polymorph is a great tool to use against enemies who appear as if they pose a threat to you, particularly ones that aren't in any major melee area and Counterspell as well as Frost Nova can also be extremely annoying for enemies.
Your role in PVP in groups is very similar to your role the role of PVE. Your primary goal is to quickly distribute dmg and making it challenging for the opposing team with the use of counterspells, polymorphs and frost Nova. Because you're likely of having multiple players employing AE spells like arcane blast is definitely the best option. However, using ae to spam is a guaranteed way to get the attention of the enemy force.
You'll need to search for three distinct kinds of equipment and being one of the significant (and the most expensive) kinds being those that can increase damage to buy WoTLK Classic Gold spells that come from specific trees. E.g. there are times when you can find staffs that add six damage to all of your Fire spells or similar. These are typically sold for some money in auction houses. Auction House, but are worth the effort when you can locate one that will enhance your main spell tree.