To address your questions and comments in order:
First, there is no way in the most recent WAPI version (2.0) to create a nameserver group or to retrieve a list of its members. To do this you would need to use the older Perl-based API.
Second, when you are creating an authoritative zone (either forward- or reverse-mapping) you can specify a nameserver group as the (set of) nameservers for the zone. For example, the following command
curl --tlsv1 --user 'admin:infoblox' --header 'Content-Type: application/json' --data '{"fqdn": "foo.com", "ns_group": "internal-nameservers"}' 'https://gm.example.com/wapi/v1.1/zone_auth'
creates the new forward-mapping authoritative zone "foo.com" and assigns the (existing) nameserver group "internal-nameservers" to be authoritative for the zone. (This works in WAPI version 1.1 and later.)
Finally, you cannot specify a nameserver group when creating a forward zone (which is not the same as a forward-mapping authoritative zone). This is not a WAPI-specific restriction; you can't do it in the web GUI either. When creating a forward zone you always have to individually specify the servers (including IP addresses) to which requests for that zone will be forwarded. For example, the following command
curl --tlsv1 --user 'admin:infoblox' --header 'Content-Type: application/json' --data '{"fqdn": "bar.com", "forward_to": [{"name": "dns1.example.com", "address": "10.0.0.2"}, {"name": "dns2.example.com", "address": "10.0.0.3"}]}' 'https://gm.example.com/wapi/v1.1/zone_forward'
will create a forward zone "bar.com" that forwards to the two servers dns1.example.com (with address 10.0.0.2) and dns2.example.com (with address 10.0.0.3). (This also works in WAPI version 1.1 and later.)
