If you view participants from /user/index.php in a course that has separate groups set and forced, and you are not part of a group, you get the error message:
"Sorry, but you need to be part of a group to see this forum."
This appears to have been there since 1.6. The comments around it refer to a forum as well.
This may also be associated with bug number 9847.
- will help resolve
MDL-9847 metacourse, view participants error