After checking the REST API call I can see that it works using sites. Manual groups are not linked to sites. So looks like they are not going to be supported with that call.
GET:
/api/computergroup/{site}/{id}
/api/computergroup/{site}/{id}/computers
PUT:
/api/computergroup/{site}/{id}
POST:
/api/computergroup/{site}/{id}
DELETE:
/api/computergroup/{site}/{id}