Choose File | Export selected project (.aia) to my computer to export the source code (blocks) for your project. The source code is downloaded in a .aia file.
If you send it to a friend, they can open it with File | Import selected project from my computer.
Create a file that can be installed in a device (.apk) by going to the "Build" menu on the App Inventor toolbar.
Select "Application (Save to my Computer)." A pop-up box should alert you that your download has begun.
Once your build completes, you can email the app to friends who can install it by opening the email from their phone. If you want to distribute it more widely, you can upload it to a website that both you and your friends can access. Note that people installing your app may need to change the settings of their phone to allow installation of non-market applications.