Matt, thanks for the detailed feedback, but it's even simpler than what you describe.
You log-in with your email address, which is your user identity. From there on, you have all your workgroups available forever on the app/website.
When you create a new workgroup and invite someone, they get a link by email.
When they click the link, they go directly to the workgroup, (because having access to the link - proves their email identity). One click - they're in.
So zero setup, but with a permanent and secure identity, and all your workgroups are always available to you - with full history, search, file uploads etc.
It's really like a very very simple to use Slack, with no setups or team limitation.