OpenStack - assign security group to existing instance

I want to assign a security group to an existing instance. Is this possible?

Now you can:

In the Horizon web interface provided that you have a pretty recent version of Horizon, i.e. >= Grizzly: Change security group:

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.