Accounts in My Pingdom can be invited to other accounts, on certain subscription plans, to view and administrate them. They are referred to as users on the main account.
This means they have at least a free account of their own, as well as access to the account they are invited to.
You can switch which account you are administrating in My Pingdom by first clicking the box in the lower left corner in My Pingdom to open up a new panel, covering the menu, with some new information and links:
The account you are invited to is shown as a box beside the one you are logged in to. You can click on these boxes to switch accounts: