i need to let a user to add a role from a frontend form

I think each user can only have one role.
However this is how its possible to change role, if for some reason that I dont know the user can have more than one role, remove the remove role line. you can check the available roles here.

$current_user = wp_get_current_user();
// Remove role
$current_user->remove_role( 'subscriber' );
// Add role
$current_user->add_role( 'editor' );