I think joining EMRA/ACEP and SAEM are both great ideas, I never joined AAEM. Not only does it look good on a CV, but there are awesome resources available.
EMRA- Scientific assembly is dirt cheap and I would highly encourage you to attend. ACEP news is a monthly magazine from the college with great articles and CME articles. They also send med students great pocket cards (sepsis, clinical prediction scorecard, etc.) and a book written for med students when you join
SAEM- There are multiple levels of membership, from free to more expensive. I did the low membership option that gave me access to EM Select, which is an awesome tool to organize yourself on the interview trail. They also have a monthly newsletter.
Feel free to PM me with more questions.
MT