BuddyPress.org

This solution is even better because it displays the leave group button, which could be helpful, if you want to restrict users to 1 group only, but be able to leave 1 group and join another….

function action_groups_member_before_save( $array ) { 
	
	$user_id = bp_loggedin_user_id();
	$result = groups_total_groups_for_user($user_id);

	if ( $result > 0 ) {

		wp_die('You can only join 1 group	');
	}

	return $button;

	}

add_action( 'groups_member_before_save', 'action_groups_member_before_save', 10, 1 );