Saving the user id to the database. (vk api)

You need to somehow save the value of user_id, so that you can then send a newsletter to all users who wrote to this bot.

vk.messages.send(user_id=event.user_id, random_id=get_random_id(), message='Отлично. Вы были занесены в базу данных', attachment=','.join(attachments))

import vk_api
import datetime
import vk
from vk_api import VkUpload 
import wikipedia
import schedule
import requests
from vk_api.longpoll import VkLongPoll, VkEventType
from vk_api.utils import get_random_id
from vk_api.longpoll import VkLongPoll, VkEventType

vk_session = vk_api.VkApi(token='')

def register():
    #longpoll = VkLongPoll(vk_session)
    #vk = vk_session.get_api()
    #for event in longpoll.listen():
    #if event.type == VkEventType.MESSAGE_NEW and event.to_me and event.text:
    #Слушаем longpoll, если пришло сообщение то:         
        if event.text == 'Начать': #Если написали заданную фразу
            if event.from_user: #Если написали в ЛС
                #attachments = []
                #upload = VkUpload(vk_session)
                #image_url = 'https://i.imgur.com/zo7QiEo.jpg'
                #mage = session.get(image_url, stream=True)
                #photo = upload.photo_messages(photos=image.raw)[0]
                #attachments.append(
                    #'photo{}_{}'.format(photo['owner_id'], photo['id'])
                #) #Если написали в ЛС
                vk.messages.send(
                    user_id=event.user_id,
                    random_id=get_random_id(),
                    message='Отлично. Вы были занесены в базу данных',
                    attachment=','.join(attachments)
                )


longpoll = VkLongPoll(vk_session)
vk = vk_session.get_api()
for event in longpoll.listen():
    if event.type == VkEventType.MESSAGE_NEW and event.to_me and event.text:
        register()
    if event.to_me and event.text:
        vk.messages.send(
            user_id=event.user_id,
            random_id=get_random_id(),
            message='GG, well played'
            )