Yes this is certainly possible. It may go against the contract you sign with your wireless provider but that's not a technical issue of course. There have been several examples of setting this up over at the
www.openwrt.org forums. I haven't been keeping up with forums there lately so not sure if the examples are still there but it really isn't hard to set up. There are several approaches you can take. The easiest would be to create a wireless bridge between your house and their houses and put them on the same network as your clients. Of course this would be the least secure and you may want a little privacy and keep your traffic separate so you can't spy on each other. There are a few different ways to do this by actually routing between the devices rather than bridging and possibly using separate vlans. How far apart are your neighbors houses? You will probably also need appropriate antennae between all of you depending on your geographic configuration.