Joomla is built and maintained entirely by volunteers. The name comes from the Swahili word jumla, meaning "all together", which is a reasonable summary of how the project works: developers, designers, translators, documentation writers, site builders and end users all contribute.
Participation happens across the Joomla.org family of sites. The forums are the usual starting point for questions and answers. Beyond that, there is a directory of Joomla events held around the world, a directory of local and virtual Joomla User Groups, and the Volunteers Portal, which lists the project teams and how to join one.