An organisation as big as IAPS cannot be run only by the 9 physics students that make up the Executive Committee (EC). Several subcommittees in different areas exist and are constituted by IAPS members who devote their free time to help the EC on the day to day work of IAPS. These are the Subbies and you can be the next IAPS Subbie!
Several opportunities are open and you do not want to miss this chance! Volunteering and gathering experience in student associations is getting increasingly popular and it may even help you when you apply to your first job! You can be an enthusiastic beginner, a self-made pro looking to share knowledge or something in between. What is important is to have an open mind, good work ethics, the will to help and learn by doing.