First, select 'Accounts' on the left hand side. Search the account you would like to create users in and select the three dots to the far right. From the menu, choose 'manage':
After clicking "Manage" you will see you are now a level down and working in the sub account.
Now simply click on the "new user" button and fill in the fields for the user you would like to create:
Here's what the create user modal looks like:
You can simply create the user and provide them with the credentials, require them to change their password upon first logging in, or select 'Invite User' at the top to send an email invite to the portal.
Hit create and you're done!