Array formula to return list of unique items from selected columns only

Please help me figure out an array formula to return unique values from certain columns of data.

The source data is on a sheet with columns labeled for different committee groups, i.e.

Safety	Finance	Social
John	Gail	Steven
Betty	Mark	John
Bill	John	Mark
George	Steven

The same names can appear in one or more committee columns.

Another sheet includes a user-selected list of committees, populated using data validation from the range of headers on the source data.

I want to generate a list of unique names included in all of the selected committees. The user could select one or more committees, not necessarily in adjacent columns. The number of columns in the source data will change frequently as committees are formed and disbanded, so the source data would be part of a dynamic named range or table.

I know how to do an array formula to capture unique names from one column or all columns, but I'm stumped on how to capture from only the selected columns.

I am attaching an example spreadsheet with example source data and a dummy column of what I want returned.

Thanks in advance for any advice.