Nearly 60,000 scientists and engineers are currently members of Sigma Xi, inducted on the basis of their research potential or achievement. Since its founding, Sigma Xi is pleased to count more than 200 winners of the Nobel Prize among its membership. Induction into Sigma Xi has been a milestone in many distinguished research careers. Membership in this respected community of scientists and engineers is professionally rewarding and offers a chance to participate in ongoing programs and activities at the local and Society levels. Active, dues-paying Sigma Xi members also have access to valuable benefits. Membership in Sigma Xi is by invitation. Those who have shown potential as researchers are invited to join as associate members. Full membership is conferred upon those who have demonstrated noteworthy achievements in research. Each year the Society initiates more than 5,000 new members.
I feel grateful for being promoted to full member in Sigma Xi, the scientific research society, on March 2, 2012 (certificate). Their membership page has this to say about membership: