after getting access to contact, i want to do the next step.
Now I’m looking for something like a contact check like whatsapp or any other messenger app.
I just want to check, if the users from the contact book are registrated at my site (just a check, if email-address is signed up).
I know that whatsapp received many critic just because how they work with the user contact book.
So my question is:
What is a good way to check this? Is there any best practice example?
Anyone an idea?