I came across several such recommendations from professional career counselors today.
For students, Sally Kearsley, wrote an article on Jobweb, It Pays to Join a Professional Association and said, “Want to do something great for your future job hunt and your career? Consider joining a professional association--or the student chapter of a professional association! There is a professional association for almost any career field you can mention and you can join at any time, freshman to senior year (or beyond).”
Lynn Friedman gave this advice to those earning a graduate degree, “You have some basic skills but feel that you are only at the beginning. How do you continue professional development and growth? First, join a professional organization within your discipline.”
When the experts are recommending this to students, graduates and experienced professionals, it is probably a theme that you should also pick up in your membership marketing.