I am going to a wedding where all my family will be in. I am not the pro here but my family expects me to get decent shots. I am bringing my 5d3 with 24-105 to travel light with the 600ex. I was thinking using AV starting with f4 and auto iso up to 3200. Any other suggestions?
Outdoors, you can start at higher f-stop (if there's enough light). In terms of where to start indoors, if you're using the 600EX as either either direct or bounce, you should take a couple of test shots to dial in the ISO (I wouldn't go directly to 3200), shutter (for the ambient light), and f-stop. You should also dial in the 600EX with FEB (flash exposure balance). Once you have that set, you can pretty easily dial in the EV as the night goes on.
EDIT: Forgot to mention, choose what AF mode you are comfortable with, although I would recommend not using all of the points, since you never know which point it'll grab. I've found zone is quite handy, or 1+8. Easy to move where you want the focus to be around the points, and gives it an opportunity to find focus with more than just a single point.