First, you will need to access to your Dashboard account.
Only the following roles will have the necessary permissions to add, edit or disable a user:
Admin
User Admin
If you have the roles mentioned above, you need to follow the next steps:
Go to settings -> User & Permissions tab
Click on "Add a User" and a module will open with the following fields
Email: user's email that will receive an invitation to the dashboard to his/her inbox
Name: user's name
MID's: accounts (MID's) that the user will have access to
Roles: roles that will be assigned to that specific user
Click on confirm and you're all set! The new user created will receive an invitation to the dashboard. He needs to accept it and follow the instructions to set up a new password (not required to set a new password if your organization has SSO enabled)