For group assignments, the submission details page may include individual comments added to a group assignment.
By default, group assignments that award the same grade to all group members do not display any individual comments when grading group assignments in SpeedGrader. Likewise in SpeedGrader, you cannot add comments for an individual user in a group assignment.
However, the assignment details page allows you to leave individual comments for the student in group assignments where all members get the same grade. To reply to the student (and not all group members), do not select the Send Comment to the Whole Group checkbox.
Individual comments also appear in the Submission Comments filter in Conversations.