As with all Open Source projects, there are a great many ways one can contribute. The project can use help with everything from reporting bugs when found to writing new code modules.
Before reporting issues, please take a look at our thoughts on how to report a bug.
Membase consists of a set of projects, which when integrated, become the software installed on cluster nodes.
Membase, users, developers and other contributors collaborate both synchronously and asynchronously using mailing lists, forums, and chat. Home covers the various options; these are great places to find and discuss your ideas for contributions to the project.
The high-level architecture and the component overview is described in our architecture.
You can get access to the sources via our Source Repositories.
Once you have the sources, here is Building Couchbase from Source the source.
Once you have created your change, we have a section in the wiki describing how to contribute the change back to the community via getting the code reviewed.