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From Wikipedia entry:
Membership in the organization is open to anyone with an interest in its activities. Standards are developed within committees, and new committees are formed as needed, upon request of interested members. Membership in most committees is voluntary and is initiated by the member's own request, not by appointment nor by invitation. Members are classified as users, producers, consumers, and "general interest". The latter include academics and consultants. Users include industry users, who may be producers in the context of other technical committees, and end-users such as consumers. In order to meet the requirements of antitrust laws, producers must constitute less than 50% of every committee or subcommittee, and votes are limited to one per producer company. Because of these restrictions, there can be a substantial waiting-list of producers seeking organizational memberships on the more popular committees. Members can, however, participate without a formal vote and their input will be fully considered.
As of 2015, ASTM has more than 30,000 members, including over 1,150 organizational members, from more than 140 countries.  The members serve on one or more of 140+ ASTM Technical Committees......ASTM International is classified by the United States Internal Revenue Service as a 501(c)(3) nonprofit organization.