A community needs a critical mass of participation. New communities are more likely to form when viral effects are present and apply quickly. Therefore, Have Fun First so that the potential rewards of participation are apparent while one performs necessary chores.
The rewarding upward spiral of community depends on a sufficiently large population and sufficient attention within that community to the needs of each other.
When participation leads to inviting new participants a chain reaction happens. The reaction either grows geometrically or fades just as fast depending on factors such as the rate and duration at which invitations happen.
Therefore:
Design workflows for community systems so that participation inducing activities occur early in the induction process.
Encourage invitees to try a rewarding activity immediately. Design safe or weakened forms of activities create or suggest rewards without the chores related to long-term or full-power participation.
Let people use a system in a limited way without requiring a login or account creation.
Arrange experiences into a Funnel of Engagement where progressively more rewarding experiences encourage progressively more commitment to the goals of the community.
Make sure people are immediately aware that there is a community nearby that cares about what they do.
Application:
I apply this pattern in my argument for Write First, Claim Later 
as a possibly risky but growth rate enhancing induction workflow.
Local storage provides a safer mechanism for writing without privilege but without the reward of writing in public.