CouchSurfing is a worldwide social network of people who share their homes to others in the network who are traveling.

The CouchSurfing - Ann Arbor group is used for coordinating those events; there's also a weekly Wednesday meetup in the evenings, check the group for details.

Lada Adamic at the University of Michigan School of Information has a paper on trust relationships in the CouchSurfing network which describes how it works complete with graphs and math.