What's the noun for panellist? Here's the word you're looking for.
panel
A (usually) rectangular section of a surface, or of a covering or of a wall, fence etc.; (architecture) A sunken compartment with raised margins, moulded or otherwise, as in ceilings, wainscotings, etc.
A group of people gathered to judge, interview, discuss etc. as on a television or radio broadcast for example.