There is not currently a way to change meta on an existing server. It is on my to-do list to add a rax_meta module, but haven't gotten around to it yet.
You would have to manually manipulate meta via the API outside of Ansible currently.
If you want to support more than 1 group with the rax module, you would utilize the 'groups' meta key such as:
- rax:
name: foo
image: bar
flavor: 2
group: primary_group
meta:
groups: 'group_two,group_three'
etc...
The newer rax.py inventory script that is in devel right now, can handle the extra 'groups' meta key. Note that it's value right now has to be a comma separated list, not an actual YAML list (this will likely change in the future, and the rax module will just coerce an actual list to the joined CSV string.