The community group has not expressed a stance on how extensions can developed, nor has it expressed a stance on who can develop extensions.

Who can write an extension for WGSL? Can just the community group publish the extensions? Implementors? Anyone?

If more than just the community group can produce extensions, how much input should the community group have? Should we simply host a registry of extensions or a registry of curated extensions?

GL for example, has many types of extensions: KHR, EXT and vendor extensions. It does not restrict who can write extensions, though not all extensions are expected to be widely supported. Implementations can choose which extensions to support (e.g. vendor X is unlikely to support vendor Y's extensions).