Contact the Support team of your software Vendor. They will be able to send you a new invitation email.
In case you know your username, you can use the download links below.
Download link for Windows: https://shield.nexetic.com/installers/NexeticShieldInstaller.msi
Download link for OS X: https://shield.nexetic.com/installers/NexeticShield.dmg
After installation, in Guided Set-Up phase, fill in your username and then click Forgot your password? link to obtain a new password via email.
With your existing username and new password you can complete the activation.