Banshee is an open source project anyone can contribute to. This means if you find a bug you can attempt to fix it yourself, as well as add completely new features and tweak existing ones.
The simplest way to get started is to go over to Banshee's GitHub page, fork the repository and start making changes. Once you are done make a pull request and it will be reviewed and merged into the main repository.
If you don't know what to work on check out the roadmap/task list.Get the source code
If you wish to be involved with the development on a day-to-day basis you can also: