In GetCourse there is record about date when user was added to a group or deleted from it. Information is available in user card:
There is “Date of adding to a group” condition in segments.
Pay attention: if a user is added to a group by product purchase, information about it won’t be depicted. If a user is added manually, in bulk or via a process, you will find a relevant date.
If a user was added to a group before August 11, 2017, then a default date is depicted as 11.08.2017.