I'm sure there's an easy way to do this but I can't come up with it...

I have a collection with a Group string field. What I'd like to do is split that collection into (n) number of collections for each Group. I'll end up with a loop similar to this:

foreach (var group in groups)
{
   foreach (var item in group)
   {
      //process item.
   }
}

Any guidance out there??? Thanks!