Adding another site under the same account
In the left upper corner, click the current site name to expand the sites list
Click Add a new site
Follow the set-up wizard and add a payment method
Adding another site when getting an invite
In the left upper corner, click the current site name to expand the sites list
Click Add a new site
Click Accept on the top of your screen.
Overview of your sites
In the left upper corner, click the current site name to expand the sites list
Click My Sites
-> When clicking another site's name, that site's dashboard will be shown.