Google API woes

I am new (actually returning) to developing Android Apps. I was hoping to get some insight to a problem that I’ve been having.

The app I made calls the Google Calendar API and displays current appointment and next appointment, along with times and color. It all works fine. The problem I am having is this: when I pass it off to the person that asked for the app, he is met with an unusable app as it does not access his calendar

I set up a Google Calendar API with SHA1 credentials. If I make an APK for installation, will the app then just call the google cal of the user that is logged into the phone or tablet? I guess what I am asking is, how do I make this app so it is usable by other people and not just myself?

I know this is a super newbie question and probably not very well constructed but if anyone has any thoughts or advice, I would be eternally grateful.

Thanks for your help.