There are multiple ways to achieve that result, provided that your join date and renewal date fields are both fields of type date and not text fields (or calculation fields that return a text data type.) You can't compare dates to your computer's current date if the value in the field is not of type date.
If ( YourTable::RenewalDateField > Get ( CurrentDate ) ; "Current" ; "Overdue")
can be defined in a calculation field with a text result type provided that you also click the storage options button and select the "do not store" option to make it an unstored calculation.
The Boolean expression used above, YourTable::RenewalDateField > Get ( CurrentDate ) can also be used with conditional format expressions or FileMaker 13's new "Hide object when" setting to make layout objects, such as a graphic or layout text reading "overdue", appear or disappear to correctly show each member's status.
Thanks PhilModJunk - so simple and elegant. I've gone with the text display for now, but will play around with unveiling a graphic in due course. Really appreciat eyour help.