Using python and telethon it's very easy to get chat id. This solution is best for those who work with telegram API.
If you don't have telethon, run this:
pip install telethon
If you don't have a registered app with telegram, register one: The link is this: https://my.telegram.org/
Then run the following code:
from telethon import InteractiveTelegramClient
from telethon.utils.tl_utils import get_display_name
client = InteractiveTelegramClient('session_id', 'YOUR_PHONE_NUMBER', api_id=1234YOURAPI_ID, api_hash='YOUR_API_HASH')
dialog_count = 10
dialogs, entities = client.get_dialogs(dialog_count)
for i, entity in enumerate(entities):
i += 1 # 1-based index
print('{}. {}. id: {}'.format(i, get_display_name(entity), entity.id))
You may want to send a message to your group so the group show up in top of the list.