A single account cannot be accessed by multiple users at the same time. This means that you will see this message if you try to log in to an account that's already being used or logged in somewhere else. You must log out of other browsers before being able to log in.
There are some exceptions to this rule. A single browser can have multiple tabs or windows open at the same time with no issues. You can also log in to the dashboard from any number of smartphone devices.
If you would like to access your dashboard from additional computers or browsers, you can add more users by choosing the "Invite Team Members" option on your dashboard's homepage.
Click on Account > Account Management > Team Members.