Hi Graham,
This issue has crept back around.
In the Advanced Settings of ArcGIS Hub, Allow member to auto join with Email accounts is switched on, and the settings are saved. I have tried with anonymous access toggled on and off in our organisation settings (it is fixed 'on' in our community organisation with no option to change that), but when clicking a Sign Up or Follow button in a hub initiative, none of the customised messages or options show to the user. Only the following:
We would expect to see the following (preview from Advanced Settings):
Using the Sign In option on an initiative (top right) also fails to show an option to create a new account. Where we would expect to see the following (preview from Advanced Settings):