According to documentation, reusing the authentication token will result in each runner that uses it being added to the “parent” runner shown in Group → CI/CD → Runners as another actual runner with a different system ID.
Yet when I attempt to add a new runner using that authentication token, it REPLACES the runner, which is directly observable by the IP address changing on the single runner shown, instead of a new runner being added.
According to this documentation, each runner using the same authentication token should be added.
your command is using the docker executor, so I believe for you to be able to achieve what you are doing, you need to first have a runner manager using the docker-machine executor - since in the part you linked it says:
So, once the first runner-manager is registered, with the authentication token, and subsequent runners are registered also with that same authentication token, they should then be grouped as you are expecting them to be.
At least that is how I understand the documentation.