At the moment you can use Facebooks account-linking feature.
But if you have more then a Facebook Bot (Kik, Telegram, …) you have to implement your own web-linking page.
- open webview e.g. url.com/chatBotUserId=1234
- implement a signup/signin on the webpage
- connect multiple users by UserId<=>chatBotUserId
In theory easy, but in practice little complicated to do it the right way and have a cool UI/UX in the webview popup inside the Chat App.