This article examines those public interest groups that have grown through heavy advertising expenditure and sophisticated marketing techniques, most particularly direct mail solicitations. It suggests that an Olson-style rational choice understanding of this ‘mail order’ membership must be supplemented by a recognition of the importance of group activity in manipulating the factors considered by individuals in joining decisions. Thus it moves the focus from the demand-side to the supply-side. Active marketing by groups can exploit sympathy for causes and encourage the predisposed potential member to join. Individuals do consider the range of benefits offered (though the list of factors is broader than Olson indicates) in their membership choices, but the group deliberately shapes the way in which these are regarded.