How do I add a second IPHONE to my homepod

I would like to also allow my wife access to our homepod so I would like to register her iphone.

How do I do that?

Note: I am not talking about adding another 'device'. I am talking about allowing 'Home' on her iphone, with a seperate apple account to access the homepod. And I would like to know *HOW* to do it.