Ultimate Member: Activate ability to add users via registration form

functions.php

//to make this ability possible for all registration forms on your website
add_filter( 'um_registration_for_loggedin_users', '__return_true' );

//to make this ability possible only for {selected registration form ID here}
function my_custom_um_registration_for_loggedin_users( $activate, $args ) {
	if ( $args['form_id'] == '{selected registration form ID here}' ) {
		$activate = true;
	}
	return $activate; 
}
add_filter( 'um_registration_for_loggedin_users', 'my_custom_um_registration_for_loggedin_users', 10, 2 );