Auto Group depends on a selected value in a custom field

Hi

I appreciate that you made the new feature "Auto Group Joining Depending on Profile Type" recently.

I am trying to write a script to create a new user group using a value that user input ( category that this group is belong to is pre configured ) and add user to it when user input/update value in the field automatically.I hope that I can tweak the existing code that used for this new feature for my requirement.

I am wondering if you can shed a light on what file and line that I need to update to trigger a group creation and assignment of user to a group?

Many thanks

Hisako