It's useless to start sending emails to dozens of spec leaders asking them to join the group and collaborate. I know it, I've already done it before.

Instead, take a step back, think about what you are really interested in, and choose one single specification to start. Join the mailing list, find its repository on GitHub, and start watching it.

A great way to start collaboration on any open source project is with documentation. It's important, but often people involved with writing specification 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 it.

I know many people who start collaborating this way today are committed to some of the biggest open source projects.