Hopefully not a dup (I did a search and didn't find similar issues reported). Here are steps to repo:
1. Restore the moodle test case course (http://download.moodle.org/testcourse/) on your local testing moodle 1.9 instance
2. Create an user account for your course teacher role (usually I just call it teacher / teacher), assign this user as teacher of the test case course.
3. Go into the test case course and click on the "Participants" link
4. When you select "visible groups" to "All participants" and the current role to "Teacher", the teacher teacher account will show up correctly.
5. If you change the "Visible groups" to "Student Group1" (or Student Group2"), then the teacher teacher will not display (showing "Nothing to display".. See attached two screenshots.
Haven't got a chance to dig into this, not sure if this is a problem just specifically for the test case course (as it is designed for older version of moodle which has different groups codes).. but really if this is the case they ideally moodle should still guarantee backward compatibility if a course backuped in moodle 1.6 or 1.7 (with groups) would still restore correctly in 1.9 with correct groups info..etc.
Hope this helps.