Like everyone say =) get geared as much as you can via Heroics (Triumph Badges). You could get to 5k++ gs once you get everything you can get.

As for convincing them, just pst them and tell them you know 7/12 of the fights or smth, or lie a little =P like tell them you done on your main but make sure you really know the fight because sometime in PuG we ask ppl do they know 7/12 the fights but when we reach the actual fight, there is always ppl ask how to do =="

Btw whats your GS?