It's not the tris, though those are making grouping far more difficult.

You've still got left and right thigh groups in there.  In fact, the group lines are a mess!

Here's your problem in a nutshell - the thighs weld to the hip.  They do not, and cannot, weld to each other.  What I see there is the hip group overlapping the thigh group, therefore trying to move multiple groups.  Where you see the stretched tris is the thigh welding to the hip.  Where you see the break is the unwelded left thigh. 

You're best option is to eliminate the thigh groups entirely, group the bottom of the dress to the hip, then rig movement/ghost bones as explained in this PhilC tutorial: http://philc.net/Body_Handles_the_Video.php

You're very best option, though is to listen to vitters and make the dress dynamic.  Those tris are going to haunt whatever you try to do in a conforming outfit.