If you want to only add new user, you can use API /api/authz/shares/entities/grn::::stream:STREAM_ID/prepare with request {} and use it to return already assigned permissions for other users. Then use it as input, concatenate with new NEW_USER_ID using jq:
curl -s 'http://172.28.128.15/api/authz/shares/entities/grn::::stream:STREAM_D/prepare' -H 'Accept: application/json' -H 'Authorization: Basic BASE64' -H 'Content-Type: application/json' --data-raw '{}' | jq --arg user_id NEW_USER_ID '{selected_grantee_capabilities}|. * {"selected_grantee_capabilities": {("grn::::user:" + $user_id): "view"}}'
Then you can use as input for previous api /api/authz/shares/entities/grn::::stream:STREAM_ID