If two teams are tied after Group Stage Games:
a) Result of the game between the two teams
b) Goal Differential
c) Total Goals For
If three teams are tied after Group Stage Games:
a) Goal Differential
b) Goals For
If 2 or more teams are still tied after these criteria are used, then kicks from the mark to determine who advances. The WCP Organizing Committee (in its sole and absolute discretion) will decide how that's going to work if it ever happens. My guess is poorly.
Of course, what if there are three teams tied and then two of them have the same, better goal differential than the third, but when they go to Goals For the third team has a bigger total than the other two? I love these unaccounted for and madening situations.