How do I join an existing project?
Firstly, become a registered user if you aren't already one. Then, join the user mailing list for the project you are interested in. The project managers should be subscribed to this list, so you can make your case for obtaining developer access by posting to the list. If the project managers accept your request, they will add you to the list of project members. You will then be automatically subscribed to the devel mailing list for the project, and you will receive write access to the source repository.
If you feel the need for experimenting with the development environment, please use the project sandbox which is specifically intended for this purpose.