Set Variable [$ID ; LayoutTable::PrimaryKey ]
Go To layout [join table]
Set Field [JoinTable::layoutKey ; $ID]
GO to Layout [original layout]
Will add a new blank record into the portal in IWP (I think). Just don't include the freeze window step often used with non IWP versions of this script.
You might also be able to use a variation of the "checkboxes" method in this demo file: http://www.4shared.com/file/dZ0bjclw/ManyToManywDemoWExtras.html
But if you do, you'll need to come up with a method other than conditional formatting to make the check boxes change appearance to show which records are selected. A calculation field that returns a value can be used in place of the conditional format.