Agora você pode:
Na interface da Web do Horizon, desde que você tenha uma versão bastante recente do Horizon, ou seja, & grt = Grizzly: Alterar grupo de segurança :
It's possible to change the security groups on the fly using Horizon, if you're using the latest release Grizzly. Click on the "More" button for the instance you want to update and you should see an "Edit Security Groups" option.
In the CLI, use nova help add-secgroup
The code for this was added a year ago so I believe this should be available in Folsom.