It seems like the managed instance groups in the Google Cloud console should be a nice thing to use as an easy way to scale or add / remove instances. What I don't get is how to create a new instance and add it to that group? I can modify the number of instances in the Edit Group Settings page and it will create a new instance and add it to the group but I don't want a new instance automatically up and going in the group. I need to create an instance, bootstrap it, run chef-client, etc and validate that all is fine before I want the instance available in the group.
The only way I can seem to do this is to use unmanaged instance groups. This allows me to create an instance and then add it to a group.
Is there a way to do what I need using managed instance groups, since that's what Google recommends to use?
It sounds like you're trying to achieve two contradictory things:
You can't do both. You need to choose which of the above two things you want to achieve. If it's #1 that you're after, you need to have your instances come up without manual intervention (there are various different ways to achieve this). If it's #2 you're after, you should use unmanaged, not managed instance groups. The documentation is quite clear about this: