How to use the Fleet Provisioning Hook to assign a thing to a thing group?

The PreProvisioning Hook lambda is only used to authorize the device registration via Fleet Provisioning. You can define the Thing Group in the fleet provisioning template associated to the Fleet Provisioning birth certificate. You can also associate the Thing Group dynamically using Dynamic Thing Groups.

Finally, you could also associate the thing to a thing group by triggering a lambda function via an IoT Rule listening on registry events

For us, the right answer was for the PreProvisioning hook to set a variable (deviceType in the below) that the template can use to set the Thing Type. We wanted to use Thing Type in our MQTT Allow directives

    Map<String, String> parameters = new HashMap<>();
    // the provisioningTemplateBody.json uses this parameter to assign the new thing to a Thing Type
    parameters.put("DeviceType", thingType);

    return Map.of("allowProvisioning", allowProvisioning,
    			  "parameterOverrides", parameters);