I've been a member of the American Association of School Librarians (and therefore the American Library Association) for a few years now; I joined when I first started working in the media center in the fall of 2014. I think belonging to this organization is essential for practicing media specialists. After all, the AASL publishes the Standards for the 21st-Century Learner and sets the industry standard for practice in our profession. The ALA and AASL also advocate for school libraries across the country, champion intellectual freedom for all readers, and lobby Congress for legislation that supports library policies and funding. I also frequently visit the AASL website to use the standards crosswalk and to browse its best apps and best websites lists.
I'm also a member of the International Society for Technology in Education, or ISTE. ISTE also publishes a set of standards for students as well as a set for teachers, administrators, coaches, and computer science educators. While I have referenced these standards occasionally, I don't rely on them as heavily as the AASL standards. However, I have been able to get more involved with the ISTE Librarians Network, a special interest subgroup. I've posted on their discussion boards a few times, and I even had an article published in the March 2015 issue of their professional newsletter. The piece was titled "Teach by Example" (viewable on pages 9-11 in the embedded newsletter below) and discussed a one-on-one approach to digital citizenship.
I also belong to Georgia Independent School Librarians. GISL is a small organization for librarians and media specialists working at private, independent schools in the state of Georgia. They have several meetings throughout the year, mostly at metro Atlanta area schools, and run a very active listserv. I've made several contacts through this organization, some of whom invited me for a school visit when I was looking for ideas on how we plan for future library renovations. I like how this organization specifically caters to the needs of independent schools.