For QE: https://github.com/openshift/openshift-docs/pull/25837

I have tested this by deploying a new MachineSet with ServerGroupID set as a post-install operation, it works as expected, move this to verified.
$ openstack server group create --policy=anti-affinity zhsunbug
+----------+--------------------------------------+
| Field    | Value                                |
+----------+--------------------------------------+
| id       | 0d1f9449-af50-4cf5-b8a6-08dd51fca1c5 |
| members  |                                      |
| name     | zhsunbug                             |
| policies | anti-affinity                        |
+----------+--------------------------------------+
$ oc create -f machineset.yaml
$ oc get machineset zhsun13osp-s9vd8-worker-1 -o yaml
machineset.yaml
      providerSpec:
        value:
          apiVersion: openstackproviderconfig.openshift.io/v1alpha1
          cloudName: openstack
          cloudsSecret:
            name: openstack-cloud-credentials
            namespace: openshift-machine-api
          flavor: m1.large
          image: zhsun13osp-s9vd8-rhcos
          kind: OpenstackProviderSpec
          metadata:
            creationTimestamp: null
          networks:
          - filter: {}
            subnets:
            - filter:
                name: zhsun13osp-s9vd8-nodes
                tags: openshiftClusterID=zhsun13osp-s9vd8
          securityGroups:
          - filter: {}
            name: zhsun13osp-s9vd8-worker
          serverGroupID: 0d1f9449-af50-4cf5-b8a6-08dd51fca1c5

$ oc get machine
NAME                              PHASE     TYPE        REGION      ZONE   AGE
zhsun13osp-s9vd8-master-0         Running   m1.xlarge   regionOne   nova   23h
zhsun13osp-s9vd8-master-1         Running   m1.xlarge   regionOne   nova   23h
zhsun13osp-s9vd8-master-2         Running   m1.xlarge   regionOne   nova   23h
zhsun13osp-s9vd8-worker-0-f6zxn   Running   m1.large    regionOne   nova   23h
zhsun13osp-s9vd8-worker-0-kszvf   Running   m1.large    regionOne   nova   23h
zhsun13osp-s9vd8-worker-0-xscs9   Running   m1.large    regionOne   nova   23h
zhsun13osp-s9vd8-worker-1-tlnd8   Running   m1.large    regionOne   nova   40m
zhsun13osp-s9vd8-worker-1-ww8sx   Running   m1.large    regionOne   nova   40m
zhsun13osp-s9vd8-worker-1-xr4m6   Running   m1.large    regionOne   nova   40m

Thanks!