Thanks Nitesh for your response. Looks like both payload and URL mentioned in the document is incorrect.
For other's benefit, here is the correct URL & payload.
1. Add User to Role
PATCH
https://platform.adobe.io/data/foundation/access-control/administration/roles/{{ROLE_ID}}/subjects
[
{
"op": "add",
"path": "/user",
"value": "<userid>"
}
]
2. Add UserGroup to Role
PATCH
https://platform.adobe.io/data/foundation/access-control/administration/roles/{{ROLE_ID}}/subjects
[
{
"op": "add",
"path": "/ims-group",
"value": "<group-id>"
}
]