This seems like an interesting idea. Telegram doesn't allow simply scraping group members' numbers so using this method one could get good results.When you register new account on telegram you can pass up to 1000 phone numbers as your phone book. Telegram will return who is registered in TG. May be you can change this 1000 number in every login. By this way you can try to find which phone numbers are registered in TG. This way can be easily modified to targeted country\city.
This is good, but you need to know the niche to market those? There's a turnaround for that??When you register new account on telegram you can pass up to 1000 phone numbers as your phone book. Telegram will return who is registered in TG. May be you can change this 1000 number in every login. By this way you can try to find which phone numbers are registered in TG. This way can be easily modified to targeted country\city.