How to take advantage of scientific international association membership for career?
The best way to take advantage of memberships in professional societies—beyond attending conferences—is helping to organize them. This will get you in touch with the other people in your field who are at an early-career phase. (Older faculty typically don't need to, or are higher up on the food chain.) Organizing sessions is also a good way to learn your specialty better, and to keep abreast of who's working on what. 

In the long run, it can also bolster your career by providing you with "synergistic" (or "service") activities that show you to be a good member of the research community, which is important for getting job offers and promotions.