The phrase "frequent panelist" refers to a person who consistently participates as a member of a group discussion or debate on a specific topic. Full definition
Frequent panelist at conferences on film / TV, copyright, music, special events and other entertainment topics and guest lecturer at universities and colleges and industry associations across Canada (stohnhay.com)
Stephen Thompson is an editor and reviewer for NPR Music, where he writes the advice column The Good Listener, fusses over the placement of commas and appears as a frequent panelist on All Songs Considered. (npr.org)
She is a frequent panelist and speaker on current issues in contemporary art for galleries, museums and institutions. (artzine.com)