There are currently three people in the @Documentation group, including me, because I needed to join in order to create it 
Joining the group is an effective way to say yes, I am volunteering!
What we need next:
Someone to volunteer to lead / organize Documentation efforts
Others to join and commit to some time / suggest which tasks they can help with
Write up some suggested tasks and goals
Have a kick off meeting to get everyone aligned
Get work done and iterate on process, communication, and tooling.
Ideally we can get various mini groups to self organize in similar ways for tasks in the TW community. I am happy to help support a group / group lead. It can be scary to take on extra responsibilities in an open source project. Feel free to DM if you have questions.