Я решил свою проблему, вот код
from telethon.sync import TelegramClient
api_id = XXX# int
api_hash = 'XXX'
client = TelegramClient('xxx', api_id, api_hash)
assert client.start()
if not client.is_user_authorized():
client.send_code_request(bot_token)
entity=client.get_entity("@ChatUserName")
users = client.get_participants(entity)
print(len(users))
for i in users:
print(i.id, i.username, i.first_name, i.last_name)