Appian 7.3 allows us to define different 'dashboard' views for a record

Where are you defining your visibility section? Please ensure that this is defined after creating your record type > clicking on the record > Click "Create a New Dashboard " under the Dashboard section.

Yes, I can see where to enter the expression. But what data type should the expression return?

I believe it should return a boolean value. You can check if the user belongs to a group:
=if(isusermemberofgroups(loggedInUser(), <group ids list>), true(), false())

=true() doesn't seem to work

Alastair, have you checked if the expression you're executing is a valid one? Quick way to test is to make this as the definition for the default dashboard view (only for testing) and if it works as expected, you can move this to the Additional dashboard section.
The visibility requires a boolean. I have a few example on my local and visibility of true() works. If should definitely be able to view this with Admin priv with the above said set to =true()

Hi Sathya - I've used the same expression as for the main dashboard, and the visibility expression is just =true()

Just in case it helps anyone else, we've just upgraded to 7.3 and experienced the same issue; turned out that the issue was not with visibility expression =true() which works, but that the 'Name Expression' field needs to be an expression i.e. "test" rather than simply test.

We figured this out after navigating directly to the generated URL and finding the test dashboard present, but apparently without a label.

Thank you - that now works. Not the first time I've been caught out by Appian's inconsistent approach to quoting literals...