Show count instead of id in "manage groups" table

This has grown into a big pet peeve for me, and turns out to be an easy fix:

The "manage groups" screen displays the group id in the 2nd column of the table. But the id is useless information to non-developers (and developers have other ways of getting it). I've seen users frequently mistake it for a count of group contacts, saying "Why does my newsletter group only have 6 members? ... oh wait that's group id 6"

So, let's replace something useless and confusing with something useful and intuitive.