The best way to inform us about the bugs you find is using our issue tracking tool. Just open a new issue and be sure to fill in all information that can help us to find and reproduce the issue experienced by you. If possible also make sure that the problem isn't already known by searching the database before committing any new issue.
If you're interested in helping our project as a regular tester, we suggest that you contact us by e-mail. We can then provide you with information which parts of the framework could use additional testing, existing test-plans, etc.