Quassel IRC is made by many people, and we welcome any form of contribution. It is always good to get in touch with the developer community first, though.
Unsurprisingly, IRC is a good way to get in touch with the Quassel community. We have a rather large IRC channel #quassel on the Freenode network where you'll find us.
We have mailing lists now! Please check the listserver pages to find out how and where to subscribe. Please note that these lists are very young and don't have many subscribers yet.
The preferred way of contributing patches to Quassel is via our repository on Github. Simply fork the repository and then make pull requests. Note that it may take a while until those get reviewed; but they won't get lost. It's also always a good idea to talk to the developers prior to writing a new feature.
Bugs and Feature Requests
The bug and feature tracker can be found here. We could also use help with triaging the bugs and converting useful patches into pull requests on Github!
Wiki and Documentation
Our Wiki is the central repository for any form of documentation. It's mostly user-driven, so please feel free to help out!