taken from Thad Curtz's comment in the Prius Co-op? thread

At first I thought there wasn't much to tell - we just did it... But that's not quite right.

• We did it with people we've known for thirty years.
• They live about seven houses down the street from us.
• We've shared a number of other things with them over the years. We shared a lot of child care with them when our kids were little. We split a CSA share with them for a few years. We've co-owned a freezer with them for years - it's in their garage, so it's easy to get to. We've owned a several lawn mowers with them (one after another, not all at once). We're working (really slowly) on putting a mesh wireless network on our block to share a high speed Internet connection with them and other people.
• They own another car, so there's a backup when two people want to drive at once. There's also somebody else on the block with another car that it would be easy for us to borrow, if we ever had to.
• When we started this, my wife was retired and I was riding the bus to work. One of our friends was retired and one was car-pooling to Shelton, so she drove the Prius three days a week, but we knew in advance when it would be gone. Now I'm retired, and the one of us who's still working usually takes the bus. Bottom line - the four of us have probably been driving less than many people do.
• Sometimes, we schedule ahead - when somebody's taking the Prius off on a trip, or when they want to use it for some regular, repeating event. For a stretch, one of our friends used it to go visit his mother in Lacey every Wednesday morning. Usually, we just call up a little ahead of time or at the last minute; I actually can't remember a single time when I've wanted to hop in the car and do an errand and it's been a problem, but having a backup car helps insure against that.