Comment 4 for bug 1054035

I'm not sure I understand.
The way to handle this in cloud-init now is just to be more explicit with your user-data or config (/etc/cloud/cloud.cfg.d).

#cloud-config
groups:
 - foogroup
users:
 - name: ubuntu
   primary-group: foogroup

Then you'll get what you asked for.
In the past if the user 'ubuntu' did not exist, it would not be created at all.
Now, if no 'users' item is specified in config, then it expects that the default user can be created.

So, since that default user cannot be created, it is at least arguable that the right path is to fail. Creating groups not explicitly requested by the user would seem wrong.