Contacts on your device (phone numbers, first and last names) will be seamlessly uploaded to Telegram servers so you could find them in Contacts section of Telegram X, or receive a notification once they join. When you delete a contact on your device while sync is on, it will also be deleted from your contacts list on Telegram servers. You can always turn sync off or clear data on Telegram servers via Settings > Privacy and Security > Delete Synced Contacts. To turn sync on, you also have to allow Telegram X to access contacts on your device.