Add someone using phone number

Struggling to understand that one.

Do you have a second sim on that phone, or a second Google account where you switch to the second account as a different user. (now I'm struggling to understand myself)

Has their phone number always been in one Google Contacts app associated with your primary account.

Someone can help you from there.

 

"Will they see that I added them...?". No, when you add a contact to any account that person is not notified that you have done so. So they will only "see that you added them" if you contact them, and they'll only see what account you added them to if you give your phone to them and let them look through it.

 

Yeah it doesn't matter how you add a contact to your device, the contact never knows on their end if you added them.
That kind of information is your personal information only.