FieldTrip is an Open Source project with contributions from various individual users and research groups. You can contribute to FieldTrip, either by improving existing functions, or by implementing new functions.
FieldTrip is primarily developed at the Donders Centre for Cognitive Neuroimaging (DCCN). Most people at the DCCN that use EEG or MEG in their research are using FieldTrip for (part of) their analysis. Some of these people also contribute by improving and extending the code.
The source code is maintained using the git version control system. All changes to the code are automatically available for the internal DCCN users, and every evening are also available in the latest version on the ftp server for the external users. On the git and github tutorial you can read how to access the latest version of the code and how to contribute.
The source code in the FieldTrip toolbox is split over multiple directories reflecting the modular organization. The modular organization facilitates a separation between end-user functions that also do data bookkeeping, and low level functions that focus on the algorithms.
For many of the tools (github, wiki, mailman, twitter, facebook) that we use we have made an integration, e.g. an update on the wiki is automatically tweeted.