If you have User access in multiple TaxJar accounts, you can easily switch between those accounts.
When you log in, you can see the account name you're currently viewing in the Upper Right Corner of the Navigation Bar:
To switch accounts, click the account name and select "Switch accounts" from the drop-down menu:
The next screen will show you a list of accounts that you have permission to access and you can switch to one of those accounts by clicking the "View Account" button: