It's been a long time since I've read that book so I will not be able to give you textual evidence, but I can give you the answer.
The answer would be that they see each other in secret and are seeing each other at night time. Romeo goes out to Juliet's house and whisper's "Juliet" and she hears him and whisper's back "Romeo, where art my Romeo". So, they are hiding from their parents, and everyone else to not get caught.
Hope this helps :)
