The new signupdialog macro is designed to be called from a node of type html.
cookieDomain -- determines what domain membership will be for.
htmltext -- is displayed if the user is signed in.
membershipkind -- the same three values as in the membership prefs panel of OPML Server -- closed, limited, open.
invitecode -- the invitation code that's required if the membership kind is limited.
I'm using it to prototype a new site I'm working on.