After Google closed down App Inventor at its end and gave the project to MIT, it is available for public use for the first time. Still in beta, MIT App Inventor has already gone through two phases of testing and the current public beta will help in cleaning up rest of the bugs.
All you need is Google ID to access App Inventor and you can start building apps. As the software is beta, make sure to backup you apps at regular intervals.
If all goes well in the public beta, it will soon become a fully stable MIT service.