Below is a step by step guide on how to accept a Landscape invitation for the first time.
WARNING: this guide is slightly outdated in the sense that the Launchpad screens should be replaced by login.ubuntu.com SSO ones. But the same idea applies, i.e., an external authentication mechanism (Launchpad before, login.ubuntu.com SSO now) is being used.
Click on the invitation url
Follow the link
Register with Launchpad
Check inbox for Launchpad e-mail confirmation
Click on confirmation link
Complete Launchpad registration
Complete Landscape registration