See the documentation here with the examples: https://docs.mendix.com/refguide/xpath-keywords-and-system-variables
So in your case something like
[$signupdate > '[%BeginOfCurrentDay%] - 4 * [%WeekLength%]']
Create a variable $MonthStart of type Date and value [%beginofcurrentmonth%]
Use this xpath for retrieving
[datejoined >= $MonthStart]
That will return a list. Count the list with a list aggregation.
Chris nicely points out that you can use variables in your XPath expression which gives you much more flexibility.
For example you can define the following variable
$OneMonthBefore = addMonths([%BeginOfCurrentDay%],-1)
to go one month in the past. This is not possible directly in XPath because Xpath does not support the addMonths function.
The Xpath would then be
[datejoined >= $OneMonthBefore]
Hope this helps,