What kind of invitation code are we talking about? how can i find out more?
Invitation code is exactly the thing that it's name suggest. It is a code, like a password that you can receive from people that are allready part of Idena network. Verified Idena members can issue a code and invite new people into the network. You need invitation code so you can participate in next Idena validation. More about invitation codes and how to get them.
https://idena.site/faq_tutorials.php#answer11