wait.  grouping and welding are two different things.  they're not related, afaik.   the constrained group could become empty without the mesh falling apart.  you just wouldn't have anything in that group.  or maybe those vertices aren't in the two dynamic groups.  yeah, i'm just not sure.  and while i'm sure we can test and find out pretty easily, and figure out of the order of making the groups matters (because i know i've had vertices leave a group if i put them into another in Poser), i was just thinking,  it's probably just easier to avoid the whole situation when it matters and make vertex groups to begin with.

oh, and i tend to get annoyed at how constrained groups distort when you morph your figure.  i like to dial spin.  a lot.  so i just use friction most of the time.  do you have a way to keep constrained groups from distorting?

oh and this is my shirt after letting it settle as a cloth sim, doing a little sculpting to add a touch of wrinkles, subdiving and smoothing it (modifiers), and giving the hems a thickness.  selected are the new seam vertices.  i added some for the shoulder seam. and in the corner is the panel in Blender 2.49 for creating vertex groups.  i probably wouldn't actually do this with a soft shirt like this, but it's not bad for demo purposes, i think.