If discussion groups are set up by section prior to the class starting, then will students that add the class late automatically be placed into their respective discussion group?
I searched Instructure's community forums for this, and it sounds like late adds are not added automatically to groups. You'll need to track who they are and add them manually.
As this impacts group assignments, it makes sense to wait until you have a stable roster before establishing groups. ALL students should be assigned to groups before an assignment is published. Grades and submissions may be affected for students who were not in a group at the time of assignment submission but who were later added to a group.
I'd suggest waiting until your roster has stabilized and you no longer accept late adds prior to setting up groups.
Vivie is right. New enrollees are not automatically added to groups! So you do have to watch until enrollment stabilizes.