It seems best suited for a group who are familiar, but not necessarily friends. A class is an excellent example; close coworkers are not.

And depending on what it is you want to say, you may have a lot to lose. Have you never taken a class with a teacher who displays blatant favoritism, or is sexist or racist enough to make a difference to you, but not to the administration? Being able to say something without an authority figure being able to punish you can be pretty important.