Final answer:
Individuals join interest groups voluntarily due to personal interests, and these groups are crucial for civic engagement and can offer purposive, solidary, or material incentives to their members.
Step-by-step explanation:
People voluntarily join interest groups due to a personal interest, and these groups play a crucial role in facilitating civic engagement by amplifying individual voices. Individuals are often motivated to join due to various types of incentives. One such incentive is the purposive incentive, where people join to support the group in achieving its goals, as seen with the increase in ACLU membership post-9/11. Another incentive is the solidary incentive, which is about the pleasure of interacting with like-minded individuals, like in the Union of Concerned Scientists. Lastly, material incentives, such as those offered by AARP, provide tangible benefits like health insurance and reduced prices on prescription drugs which attract members. Interest groups can be a form of membership organizations where individuals join of their own accord and often have common concerns or share viewpoints, as is the case with the NRA or Brady: United Against Gun Violence.