While (Cite Contribution) & Autumn Jane have given you excellent inputs to PLAN a beginning, Chandrakanth has surely pointed to the types of awkward situations that CAN arise, consequently reflecting on HR.
However, IF the suggestions given by Chandrakanth are indeed implemented, the very idea of the whole exercise being an informal one vanishes, possibly leading to some sort of unwanted or undesirable backlash.
I guess the trick would lie in 'BALANCING' the various aspects, keeping in mind the fact that this is the first time you would be having such a session. Maybe, in the first few sessions [presuming the intent is to make this a regular affair], you can focus on 'WHO' would attend the session from among the employees—if you can choose the more mature ones in the initial rounds, that would keep the discussion on an 'even keel' AND ALSO ensure the RIGHT feedback goes back to the other employees after the sessions.
In a way, you are ALSO ensuring this becomes a 'Learning' experience for all involved, with one 'fine' difference: the whole approach is CALIBRATED rather than leaving it as a 'free-for-all' right from the word 'go' OR including any sort of CONTROL in what's discussed. The selection of the participants can be made more flexible as you move along. Hope you get the point.
All the Best.
Regards,
TS