What are we doing here?
When we talk about multi-site or enabling your access to multiple sites we're actually referring to the act of pairing a user (you) with multiple sites.
Below is a graphic that shows what the difference is between users and sites.
Steps by step
Navigate to the Settings page and select the Blue "+ Add User" button:
Then, within the drawer that opens, enter the name and main email you have decided on and click "Invite User" at the bottom of the drawer (you may need to scroll).
Navigate to your email provider and click the "Accept Invite" button within the email.
Once you've done this for all Sites, you'll notice, upon login to the main user, a multi-site option will appear.