Minor typo: @scv should be @csv in the end. It works otherwise:
[Expert@DallasSA]# mgmt_cli -r true show-group name 'RFC 1918 Addresses' --format json | jq '.members[] | [.name, ."ipv4-address", .subnet4, ."mask-length4" ] | @csv' -r
"RFC 10/8",,"10.0.0.0",8
"RFC 192.168/16",,"192.168.0.0",16
"RFC 172.16/12",,"172.16.0.0",12
Note that this only covers hosts and networks. Address ranges have another set of properties, DNS domains work solely based on the name of the object, and wildcards have yet another set of properties. Groups can also contain groups, which gets really complicated to handle.
As long as you know roughly the contents of the group ahead of time, this is fine.