Add info about Terra Groups

[KatherineCox]

Perhaps as an appendix?

I lean towards leaving it out of the main Getting Started walkthrough in the interest of simplicity. So, make an appendix, and then in the main chapters add a note that, if you've got a big lab, you may want to check out these instructions instead?

We've also previously discussed making a guide that goes more in-depth about how to organize / customize stuff for your team - alternative Billing Project setups, different types of Workspaces (e.g. "Data" workspaces), etc. If we ever make that guide, then Groups could get discussed over there.

[avahoffman]

Agreed, seems lowish priority for this book.