I don't know about as an intern, but it will be necessary to join ACP by your final year of residency and stay in it for the rest of your career. ACP pretty much controls the certification/recertifiation/maintenance of certification programs for IM.
All of the specialties are moving towards having to continuously perform CME/quality improvement to maintain your certification in good status (you can't do it all in the 10th year anymore). Although technically possible to do this without being a member of the ACP, it is a lot easier to do it through the ACP- or if you are absolutely opposed to being a member, paying a higher non-member fee to do the activities at the annual meeting.