If you are using FileMaker 10 or 11 you can use script triggers to do this.
If your member type field is formatted as a pop-up or radio button field, you can use OnObjectMOdify. If you use a dropdown list, you can use OnObjectSave or OnObjectExit.
Your script can simply change layouts to reveal the portal, it can be popped up in floating window or you can hide the portal behind an invisible tab control and your script can use Go To Object to select the tab that reveals the portal to your join table.
If [ YourTable::MembershipType = "Organization" ]
Go To Layout [//specify layout with portal]