Firstly you need to add to the account subscription any additional seats required. you can do this by navigating to the Subscriptions tab seen in the screenshot below
Once in the Subscription tab select the relevant plan and then the number of users you wish to have on the account.
Then you will have to navigate to the Admin tab, then into the user's tab
Once you are onto this page, please navigate to the top left and select the "Create User" button. this will take you to a quick for that asks for some information about the new user, it will also let you set them a password and username which they will use as their MyApiary logins. Please see the User Permissions guide for more information regarding roles and permissions.(Note, you must be an admin user to access this feature.)