In general, labor union organizers tend to try to entice employees to join unions by persuading them that it is in their best interests to do so. This is particularly true when a union is trying to organize in a given workplace. If the union already exists in a workplace and is simply seeking to get more people to join, it is possible that the persuasion might also include more high-pressure tactics than is the case in a unionization drive.
In a unionization drive, the main thing that a union can do is to persuade workers that they will be better off in a union. The union organizers try to argue that the union will be able to bring the workers better pay, better working conditions, and more job security. In these cases, there really is nothing else that the unions can do because they have no leverage over the workers. If the union already exists in a workplace, it can exert more peer pressure on employees who are not yet members.
For the most part, though, labor unions seek new members by arguing that unionization will lead to material gains for the workers.