This article looks at the emergence and maintenance of interest groups. We systematically identify a range of membership incentives that underpin mobilisation and apply them to a case study, the Federation of Small Businesses (FSB), focusing on the recruitment literature rather than the special niche literature on trade associations. The FSB was created in 1974 and membership was 'flat' at about 40,000 until recruiting method changes were introduced in the early 1990s. The total now approaches 200,000. This account of FSB recruitment argues unsurprisingly that the group resulted from a combination of diverse influences rather than the Truman type of 'automatic' mobilisation. More surprisingly, it notes that while early membership is accounted for by the perspectives of Truman, Salisbury, Wilson, Moe, Opp and Sabatier, the major increases of the past decade requires attention to the contribution of face-to-face marketing and Olsonian incentives. The FSB is an unusually good example of group entrepreneur activity that illustrates how entrepreneur incentives are different from those salient to members. Groups can experience different phases and forms as they grow. We exploit a 'natural experiment' by illustrating how the FSB addressed membership stagnation by changing the recruitment package in a broadly Olsonian direction. The 'before and after' patterns of recruitment allow a 'test' of these techniques. It also raises the issue of how different recruitment efforts produce a membership with different political characteristics.