Click on Settings (gearwheel in the upper right corner) and select Users. Here you will see who is currently registered as a user. To add more users, enter their email address and click on Invite. The invited user will then receive an email with a link to a page where they can create their own user account. 

In case the link in the email does not link to the correct page, the new user may need to log out of possibly already existing user accounts on Mynewsdesk. 

If, however, the new user already has a press officer account with access to another newsroom on Mynewsdesk, there will also be an option to log in on that account, in order to now administer several newsrooms via the same already existing user account.
Please note that this requires a press officer account. It will not work to log on a journalist account, which you use to set up alerts on what is being published on Mynewsdesk. 

You can invite an unlimited number of users, and there are no additional costs.

The contact details that are put in when creating a new user account will never be displayed publicly in the newsroom. Publication of contact information in the newsroom is carried out independently of the press officer’s user account, in the menu under Publish > Contacts (see where do I add or edit a contact on my public newsroom).

Please also see:
What is meant by Account Owner?
What is the difference between a journalist account and a press officer account?