TWO problems.
You should not be sharing an Apple ID with anyone. Apple IDs are personal; they shouldn’t be shared any more than you would share a toothbrush. If you share an Apple ID with someone else many bad things will happen. Each user should have their own Apple ID, then you can create Family Sharing→What is Family Sharing? - Apple Support. That way you can share what you want to share and not share what you don’t want to share.
It is OK to share an Apple ID among devices used by one person, such as an iPhone, an iPad and a Mac; in fact, it is a very useful thing to do. But not between devices used by different people.
Here is Apple’s FAQ: https://appleid.apple.com/faq/#!&page=faq. See the 4th bullet item.
2.
You can’t add storage to an iPhone; it will never have more storage than it did when you bought it. What you bought (or actually rented) is iCloud storage, which is used for syncing and also for backing up. But it doesn’t add any capacity you your phone. See→What's the difference between device storage and iCloud storage? - Apple Support for details.