There are several IAB member categories:
IAB General Members are corporate entities or standalone divisions of a corporate entity whose revenue is significantly based on the sale, delivery or optimization of digital advertising or marketing campaigns.
Any company that does not qualify as a General member may join the IAB as an Associate Member, provided it supports the media and marketing industries.
Long Tail Members
Independent sites and companies who act as publishers (selling indirectly, through a network, or directly) and have revenues under $1 million per year.
Read more about the Long Tail Alliance Events