The invite only option means that people must send a request to you to join your alliance, as opposed to the open option which means anyone can just add themselves to your alliance. If you want to ask people to join you, you simply do so by word of mouth (e.g., if you know people, just ask them); or you can head to the Alliance Gate forum and start a recruitment post giving some info about your alliance. Then they can send you a request to join you, if you keep it invite only, that you either accept or decline in game. Hope that helps! Good luck!