There are several IAB member categories:
General Members
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.
Associate Members
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.
Startup Members
Pre-revenue and startup companies, whether they fall under associate or general membership, are less than two years old (from incorporation) and have less than $1 million in revenue.
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.