When you activate an invitation to join Endpoint Cloud, you are given an account automatically. The account has a role, either Administrator or Security Officer.
If you are given an administrator account, you can access all features in Endpoint Cloud but you cannot download and register the AD Connector.
Only people with the security officer role can download and register the AD Connector.
You can see your role by looking at the Administrators section on the Settings page (see Administrators - Settings Page).