Add a description of groups where we maintain lists of contributors/team members.

docs/project/groups.md

@@ -0,0 +1,30 @@
+# Carbon: Groups
+
+These are groups used by the Carbon Language project, listed here for central
+tracking.
+
+We use a mix of:
+
+- **GitHub teams** to assist contacting key contributor groups on GitHub.
+- **Google groups**, usually as Google Drive ACLs. We generally won't use these
+  as contact lists, unless specifically mentioned. Please prefer Discourse
+  Forums.
+
+## All contributors
+
+- **[GitHub organization](https://github.com/orgs/carbon-language/people)**:
+  Grants GitHub access.
+- **[Google group](https://groups.google.com/g/carbon-lang-contributors)**:
+  Grants Google Drive access.
+
+## Core team
+
+- **[GitHub team](https://github.com/orgs/carbon-language/teams/core-team)**: A
+  contact list.
+
+## Review managers
+
+- **[GitHub team](https://github.com/orgs/carbon-language/teams/review-managers)**:
+  A contact list.
+- **[Google group](https://groups.google.com/g/carbon-lang-review-managers)**:
+  Grants team-specific Google Drive privileges.