The microphone should have a mute facility to allow private conversation. May only be possible for one to talk at a time - cue taking turns. Background colours should be mid tones. Avoid reflective surfaces. Clothes colours need to be muted, pastel or dark, plain.
Sound insulation is important in the room and preferably with no external noise.
Good levels of lighting - blackout any windows to avoid changes in the light through them. Managing a session can be difficult - decide how to cue people if not a voice activated camera. Some face to face contact is desirable also. Users need some training - they need a mental model of the system to understand the various parts and how they interact. Presenters should be aware of distracting mannerisms - fidgeting, shuffling papers noisily.