Why is it so that Socratic does not have specific groups of contributors for each subject?
Why are you asking to form designated groups at the first place? I think there are very regular contributors, occassional writers and few irregular visitors; but most are responsible and decent teachers. Moreover there are moderators and heroes.
I think this platform is dynamic and already promotes group activity.
When you read an answer written by someone else and decides to add few more relevant points, you can do so by editing the answer.
Now as you open this specific answer to read, you will see that there are two contributors, Brian M and 1 other. If you click on it, you will even get to know the amount of contribution each of us made.
So groups of contributors are spontaneously made here on socratic.