You can subscribe people to your bot in different ways. For example, you can use a generated link to your chatbot and place it anywhere you want or you can use Facebook Ads to find and get subscribers.
If you have a website, you can add a button there, modal or bar widgets, customer chat, etc.
If you don't have a website, but you want to use Google Ads, you can land customers upon landing, provided by Manychat.