It's only possible to have one account per company, and each company is set up individually in Mynt. You can be administrator for several different Mynt accounts, then you can easily switch between them in the web environment.
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Login to mynt.com and place the mouse pointer on your name at the bottom left of the view. In the box that appears, place the mouse on the company name and then switch between the different companies for which you are admin.

Note that you can only switch companies if you use Bank-ID as log in method. If you use email and password login, login in with the email and password that you have created your account with for that company.