Perhaps a matter of semantics, but I don't criticize, frank or otherwise. To give criticism implies I know what's right and the creator of the photo does not. I will say what I like, or ask a question (like why did you choose this level of contrast), or in a few cases say what I would have done. But that's not to say I'm right and they're wrong.
So, in a group dynamic, it's about exploring what choices (composition, exposure, contrast, etc) others would have made to allow the creator of the photograph to decide whether he/she feels those choices add or detract from the photograph.
An exemption to this are technical details (for example, in or out of focus) that can more easily be put in a right vs. wrong category.