When all group members click the [Complete Assignment Submission] button, the submission status of all group members changes to 'Submitted', and the submission file of the last group member would be recognized as the final group assignment file.
(Note) You should first set ‘Assignment Settings > Submission Settings > Require students to click the submit button > Yes’ to set ‘Assignment Settings > Group Submission Settings > Require all group members submit > Yes’.
All group members' submission will be considered as completed by the assignment which a random group member has submitted.