Create AWS Greengrass group and core.

greengrass_group_create.py

import json
import yaml
import boto3

gg = boto3.client('greengrass')
iot = boto3.client('iot')

group = gg.create_group(Name="my_group")

keys_cert = iot.create_keys_and_certificate(setAsActive=True)
core_thing = iot.create_thing(thingName="my_group_core_1")

iot.attach_thing_principal(
    thingName=core_thing['thingName'],
    principal=keys_cert['certificateArn'])

core_policy_doc = {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": ["iot:Publish", "iot:Subscribe", "iot:Connect", "iot:Receive", "iot:GetThingShadow", "iot:DeleteThingShadow", "iot:UpdateThingShadow"],
            "Resource": ["arn:aws:iot:" + boto3.session.Session().region_name + ":*:*"]
        },
        {
            "Effect": "Allow",
            "Action": ["greengrass:AssumeRoleForGroup", "greengrass:CreateCertificate", "greengrass:GetConnectivityInfo", "greengrass:GetDeployment", "greengrass:GetDeploymentArtifacts", "greengrass:UpdateConnectivityInfo", "greengrass:UpdateCoreDeploymentStatus"],
            "Resource": ["*"]
        }
    ]
}
policy = iot.create_policy(
    policyName="my_core_1_policy",
    policyDocument=json.dumps(core_policy_doc))

iot.attach_principal_policy(
    policyName=policy['policyName'],
    principal=keys_cert['certificateArn'])

initial_version = {'Cores': [
    {
        'Id': core_thing['thingName'], # Quite intuitive, eh?
        'CertificateArn': keys_cert['certificateArn'],
        'SyncShadow': False, # Up to you, True|False
        'ThingArn': core_thing['thingArn']
    }
]}

core_definition = gg.create_core_definition(
    Name="{0}_core_def".format(group['Name']),
    InitialVersion=initial_version)

# SAVE created entities to the file.
# You'll thank me for that when it's time to clean things up.
state = {
    'group': group,
    'core_thing': core_thing,
    'keys_cert': keys_cert,
    'group_ver': group_ver,
    'core_definition': core_definition,
    'policy': policy
}
    
with open('./state.json', 'w') as f:
    json.dump(state, f, indent=4)

Thank you! I figured that out and codified in in greengo: https://github.com/dzimine/greengo