Platinum 6 includes a separate module for membership services - including address books, profile editor and reminder service. The format of these membership tools presumes expansion of available services in future versions.
The full featured sample of member services linked to card composition page can be seen in compose2.html. You can use this page as template and simple copy the setup from there and not worry about exact coding.
However if you are an experienced webmaster you may want to know all details. To invoke membership services login you should add the following code to your composition page (or any other page)
if (box && box.open && !box.closed) box.close();
Add to <BODY> tag
Somewhere within the <FORM> </FORM> tags
<input type="button" name="ab" value="Use Address Book" onClick='openab();'>
Note that <FORM> tag should contain name="compose" as in <FORM name="compose" action... >