It's useless to start sending emails to dozens of spec leaders asking them to join the group and collaborate. I know—I've done this before.
Instead, take a step back, think about what you are interested in, and choose one single specification to start. Join the mailing list, find its repository on GitHub, and start using it.
A great way to start collaboration on any open source project is with documentation. This is important, but often, people involved with writing specifications and coding don't have enough time to go deeper into the documentation. So, they are usually glad when somebody else is willing to do so.
I know many people who started collaborating this way that today work on some of the biggest open source projects.