One of my clients recently asked me whether joining professional groups could help her application. She also wanted to know if joining both the American College of Physicians and the American Academy of Family Physicians would help or hurt since they're focused on different specialties.
First, I think that it's most important to question why you are joining a group. Are you still in medical school and you want to learn more about residency in a certain profession? Great; you should definitely join! Are you a graduate who has decided on a field of interest and you want to interact with other professionals? That's another great reason to join.
Or are you only joining this group in order to list it on your CAF? If so, that is a bad reason and you might not want to do this at all. Padding your resume with groups that you've joined simply to impress program directors might actually cause you trouble. What would you do during an interview if someone asked you "Why did you join this group?" You'd have to say that you only joined to strengthen your application. That's not enough.
So before you join, think about what you can get out of these groups. Are you planning to attend conferences, read their journals, collaborate with or learn from members, make professional connections, and learn more about those fields? If you can honestly answer "yes" to some of those, and you will be able to comfortably describe your involvement if asked about it during an interview, then joining might be the right idea after all.
As for joining two different associations, remember that anything you list on your CAF will be seen by all programs you apply to, so FM programs will know that you joined the ACP, and IM programs will know that you joined the AAFP. Is that a bad thing? Not necessarily, since many people openly apply in both FM and IM, and both give you the potential to explore what it means to be a physician in a society increasingly concerned with primary care medicine. But if you're applying in both surgery and psychiatry, but don't want the program directors to know that you're doing that, then listing the American College of Surgeons and the American Psychiatric Association on your CAF might hurt you.
As always, please contact me directly, and I can discuss your specific application during a free consultation. There's no single answer to these questions or for any issues relating to your residency application; let me come up with a customized approach for you.