FR: indicate when a new tailnet is being created

### What is the issue?

Tailscale's support for organizations seems to be designed for companies and other formal organizations that hand out email addresses from a single domain. It doesn't work well for families and similar organizations where everyone has their own email address. For that use case, each person should create their own tailnet, add their personal devices to it, and then they can share devices across tailnets.

This seems ok once you know what to do, but it's rather confusing for a beginner who has a different mental model of what an organization is.

It's especially confusing for Android devices, because the app has a minimal user interface.

Suggestions:
* The Android app should display the name of the tailnet that you're currently connected to.
* The Android app should tell you when it auto-creates a tailnet.
* In the admin interface, when inviting a user, it should explain what this actually does. (Is it useful for personal tailnets?)

### Steps to reproduce

* Created a tailnet using my gmail address (on a Mac)
* Used the admin console to invite a user with a different gmail address
* Other user joined and appeared under Users
* Added an Android device that belongs to that user

What I expected: device should show up since I added the user to my organization (or so I thought).
What happened: device doesn't show up, no error message. Resolved with Google searches and doing some experiments.

What's really going on: the Android device auto-created a different tailnet for that user. This isn't obvious because the Android app's UI doesn't say which tailnet it connected to. (You're expected to know that every gmail address automatically gets a different tailnet, but the UI doesn't say that.)

### Are there any recent changes that introduced the issue?

_No response_

### OS

macOS, Android

### OS version

Mac Ventura 13.5.2. Android: I don't remember but it was a very old tablet

### Tailscale version

1.56.1 (Mac App Store) Android: I don't remember

### Other software

_No response_

### Bug report

BUG-12439281ece04f127884bf2a5c33f95ebb41666391cd0957b0a6d90d55d087be-20240103182931Z-25764461ba270ab1