Phing is under active development and there are many things to be done. The project will also welcome non-coders to help keep the documentation up to date. If you don't already know about DocBook participating in the documentation is a great opportunity to get experience!
To get involved start by doing the following:
Read this manual to understand Phing ;-)
Go to http://phing.tigris.org and subscribe to the
dev mailing list (this is usually a low volume, high quality
Visit the Phing website (http://www.phing.info/) and look for open bugs / tickets
...and of course, start to actively participate in the development by forking the repository (see below)
As of 1 January 2012 all Phing development is based on Git and the project is hosted at GitHub (https://github.com/)
In order to participate in the development you will only need to follow three basic steps
Register a free account at GitHub
Clone the Official Git repository
Read up on the (very well written) documentation at GitHub on how to setup your own repository and do things like cloning an existing repository and creating pull requests asking the official Phing maintainers to take in your proposed additions/changes.
The chances to have a change set accepted greatly increases if you adhere to the following recommendations
Follow the naming and coding principle used by Phing
Make sure you have added documentation for all your additions, including examples.
Make sure you have added unit-test code as needed
Be polite in all communication!
If you have not worked with Git before and are coming from subversion there is a bit of re-adjustment needed. Fortunately there are several SVN-To-Git re-learning guides available (for example http://git.or.cz/course/svn.html which might make the initial transition easier.
However, it is probably best to forget about your mental picture on Subversion and realize that Git is a different animal. So trying to think of everything in terms of Subversion is not really helpful in the long run. You should therefore take the time to read the (free!) book "Pro Git", by Scott Chacon available from http://progit.org/.