Only users with permissions set to 'Yes' for "Can Manage the Account and Users" can add users to the account. See image below:
If you don't have the proper permission to add users in your account, consult your account administrator or reach out to support (firstname.lastname@example.org).
To add a user, login to the dashboard and go the Accounts > Users menu.
From there, enter in the email address for the user and click, "Add User". The new user will receive an email so they can set their own login password. Once that is done, the new user can begin accessing the dashboard.
When the user is added you'll need to set appropriate permissions by clicking the Yes/No toggle.