SSRS How do I get CountRows to work?

As usual Microsoft's documentation sucks. So how do I make CountRows work in SSRS 2012? Trying to count the rows in a table by group. THe group name is Address. =CountRows("Address") gives an error:

The value for expression for the text box 'Textbox 29' has a scope parameter that is not valid for an aggregate function.

So what is the REAL way to get the count of rows.