All interest groups need to recruit and retain members in order to achieve their political goals

All interest groups need to recruit and retain members in order to achieve their political goals.  Explain what the so-called free-rider problem is and why it makes recruitment and retention difficult for interest groups.  Discuss how successful interest groups are able to overcome the free-rider problem.  In your answer, be sure to define the different kinds of selective incentives interest groups provide to their members and to mention some examples of benefits provided by various organizations.