By typing in the numbers, or selectively sharing them from the address book. This works fine on Signal, Telegram and Threema. Only Whatsapp makes it so that you have to share your entire address book with the app.
With some workarounds you can actually use whatsapp also without giving it access to your address book, which shows that it is clearly an intentional dark pattern by Facebook to make people share their entire address book with them to avoid the hassle.