The series comes from the 'Current Employment Statistics (Establishment Survey)'
The source code is: CES8081300001
Source: US. Bureau of Labor Statistics
Release: Employment Situation
US. Bureau of Labor Statistics, All Employees: Other Services: Membership Associations and Organizations [CES8081300001], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/CES8081300001/, July 30, 2015.