The specification work at REST-* aims to create RESTful interfaces for common and traditional middleware services like transactions, messaging, workflow, security, and management. They may borrow from other standardization efforts or may produce their own unique way of doing things.
Each specification is created with open source processes and open source licenses. While creating the specification is a group effort, editing and publishing the document may require a "contributor agreement". This agreement will simply say that you agree that all your submissions are licensed under an open source license. Open source organizations like Apache, Eclipse, and JBoss.org all have agreements like this before you obtain commit access to their projects. Joining the mailing lists of each specification will not require such an agreement.
Each specification may have zero or more extensions. Extensions are edge cases that didn't make it into the main specification, but are important enough to standardize and provide a certification test for.