There are two main ways that you can contribute to the CrypTool 2 project. Your first option is to just get in touch with us to give us some feedback or advice, or to offer anything else that you would like to add to the project. The other option is to contact us to set up a long-term working relationship. Both options are greatly appreciated by us; the latter might bring you on the team list after some time.
If you want to develop a plug-in for CrypTool, you can simply download our source code and start programming. There are no restrictions other than you cannot upload your results to our code repository. If you think that it is good fun and you would like to contribute your code, just get in touch with us so that we can give you write access to our versioning system.
For anything else, please just drop us a message and we can start discussing. When you get in touch with us, it would be great if you could also tell us something about yourself - where you are from, how heard about CrypTool, how you use it, and of course what your reason is for contacting us... ;-)
Further documentation and links can be found here.