Logging in as a Guest User is disabled by default.
To enable the guest login in TeamCity:
Navigate to the Administration | Authentication page.
Select the Allow login as guest user option.
Save your changes.
The Log in as guest link will appear on the Log in to TeamCity page.
By default, the guest user can view all the projects. To customize which projects a guest user has access to, click Configure guest user roles under the Allow login as guest user option and configure the respective roles and permissions. This link appears only if the per-project authorization mode is enabled.