You want to use the method WP_User::set_role() https://developer.wordpress.org/reference/classes/wp_user/set_role/
If you want to add the role as an additional role, replace set_role
with add_role
add_action( 'frm_after_create_entry', 'inactive_to_member', 20, 2 );
function inactive_to_member($entry_id, $form_id) {
if ( $form_id == 21 ) { // form id of the form to copy
$user = wp_get_current_user(); //get logged in user
if ( !$user ) {
return; //don't continue if user doesn't exist
}
// Get user roles
$user_roles = $user->roles;
// Make sure they're not an administrator
if ( !in_array( 'administrator', $user_roles ) ) {
// Make Role Census Club
$user->set_role( 'census_club' );
}
}
}