I personally think Adam Grant’s theory of givers, takers and matchers would provide some insights here. Givers would voluntarily join the movement if they believe they are giving to people and helping others; takers will need to see what they can take from joining the movement; and matchers will join the movement to give if they believe they were given before and now they can pay back; or they need to be helped in the future and now they are helping in order to get some future help in return.

Ask yourself what the people you want to move are, takers, givers or matchers? The WIIFM message will vary from people to people. It depends on the people you want to move.

At the same time, despite of the differences in personality, people will be commonly moved by motives that are selfless, kind, noble, other-focused, and fair. This is because when it comes to defining what is good or bad, people of the same or different cultures around the globe share many similarities. These similar definitions shape the fundamental belief systems of people. Speaking to these beliefs will most likely move people, regardless of their cultural background or personalities, to take actions voluntarily, such as joining a movement.

I agree with Clara points above. She has explained it very well with Adam Grant's theory.
People who voluntarily join a movement are mostly self motivated or motivated through any incident of their life. They are also influenced by their friends or colleagues who've previously joined such movement voluntarily. It's not about What's in it for me for these people because they are more satisfied and happy when their contribution to such movement leads to some positive aspects.