Add a property for projects: "Requires signed CLA"
The official TYPO3 v5 / FLOW3 projects require a signed CLA by all contributors.
It would make sense that these projects can be tagged as "requires signed CLA" by setting a property for that project record. This information would be displayed where user can apply for joining the project.
The "Join this project" button should only be shown if the logged in user has the "signed CLA" flag set. Otherwise some information should be shown how the CLA procedure works.