Privileged API user cannot add members to a group

A privileged REST API user for a CO cannot add a member to a CO Group.

An example curl call is

curl \
--verbose \
-u 'co_2.rest_user_01:XXXXXXXX' \
-X POST \
-H 'Content-Type: application/json' \
-d @co_group_member_add.json \
'https://registry-dev.skoranda.scgvm.net/registry/co_group_members.json'

with
$ cat co_group_member_add.json
{
"RequestType":"CoGroupMembers",
"Version":"1.0",
"CoGroupMembers":
[
{
"Version":"1.0",
"CoGroupId":"35",
"Person":

{ "Type":"CO", "Id":"10" }

,
"Member":true,
"Owner":false
}
]
}