WECG consists of members from each of the major browser developers. Member chairs are held by Timothy Hatcher of Apple and Simeon Vincent of Google. Current participants include employees from Apple, Mozilla, and Microsoft.
The WebExtensions Community Group has two goals: Make extension creation easier for developers by specifying a consistent model and common core of functionality, APIs, and permissions; and Outline an architecture that enhances performance and is even more secure and resistant to abuse.
The group doesn't want to specify every aspect of the web extensions platform or stifle innovation. Each browser vendor will continue to operate independently with their own policies. Developers and browser vendors interested in contributing to the group can join via the W3C website. The WECG has a dedicated GitHub repository with the community charter and work.