Producteev requires you to create a personal network before joining any invited networks. Once you create that personal network, it will show you that you have invitations to accept for your invited networks (these are usually seen in the upper-right corner next to your name.)
Can you give that a try and let me know if it works for you?
I created a personal network and now when I go back to join the original network, it says my email is invalid or in use.
I don't see any pending network invitations for you. I've looked throughout, is it possible you can ask the network owner to resend your invitation? There should be an option in their network admin panel to resend previous invitations.