This community discussion Make apps with more people talks about several ways you could share an app project.
Team members can also log in to the http://code.appinventor.mit.edu/login/ server if they share the reenter code. If you share a Project this way, people sharing the project cannot access the Project simultaneously , Those who share must be very careful to save Project copies etc. and coordinate usage. This technique could work but it has its own issues. The suggestions in the link might be a better way.