First, to add new users, you must have upgraded your account to the Team, Agency or Enterprise plan options. The Free account only has a single user option. You would upgrade because you want to manage workflow better and have multiple users with trackable workflow.
To add a new user, check that you are logged in to the brand you want to add to if you have access to more than one brand. Then click on the Cog icon in the left-hand navigation bar, and go to the Account Management page, as per the image below:
Then click on the "Users" option, to see a list of all existing users. Double check that the new user does not already exist, then click on the "+ Add User" button to add a new user, as per the image below:
Create each additional user by adding their name and email address, then select a user authority level in accordance with the permissions you want to give them. When you complete the process, our system will send an automated email invitation with instructions, to help them log in and activate their user account.