In Mercy Island, Right near the Dot on the map for Darwins Landing (halfway from the Base to Mongoose) you'll find Fortunata Fateweaver. She is an NPC who can alter your Reputation, which makes the instances you do harder or easier.
By default everyone starts at the easiest Rep. Increasing your Rep makes the MOBs in instances harder as you go up the Rep tree.
For instance, I picked the hardest Rep for me and a fellow Mastermind, and we were constantly finding Purple +3 lvl mobs in our instances. This is a great way to get massive XP when you have a friend to help out.
You can change this "on the fly" if your friend bails and you need to lower the difficulty. Just go back to her and adjust your Rep to a lower setting, this will instantly cause any instances you have to adjust in their difficulty.