Sorry for the delay in my replying…
I made a custom capability called “wpb_moderator” in User Role Editor, and then I wrote:
add_filter( 'rtmedia_media_edit_priv', 'custom_role_rtmedia_media_edit_priv' )
function custom_role_rtmedia_media_edit_priv( $flag ) {
if ( current_user_can('wpb_moderator') ) {
$wpb_moderator = true;
return $wpb_moderator;
}
$wpb_moderator = false;
return $wpb_moderator;
}
…and it returns this error:
Parse error: syntax error, unexpected ‘function’ (T_FUNCTION) (Referring to the line that says “function custom_role_rtmedia_media_edit_priv( $flag ) {”
Any ideas? 
Sorry for so many support requests this week, the sites nearly ready to launch just need a few things buttoned up.
Ben