  A sharing community prospers when participation and contribution are both
high. We suggest the two, while being related decisions every peer makes,
should be given separate rational bases. Considered as such, a basic issue is
the viability of club formation, which necessitates the modelling of two major
sources of heterogeneity, namely, peers and shared content. This viability
perspective clearly explains why rational peers contribute (or free-ride when
they don't) and how their collective action determines viability as well as the
size of the club formed. It also exposes another fundamental source of
limitation to club formation apart from free-riding, in the community structure
in terms of the relation between peers' interest (demand) and sharing (supply).
