There are actually two ways in which this process can be done. First option would be when registering for an event, and the other option would be by going to "My organization" from the menu.
When registering for an event we have the option to add a user:
Step 5 of the registration process will let you add a user.
After clicking Add New User, another box will pop up and ask you to put down their credentials.
Where it will also ask you if you want to manually set their password, and if you don't, they will receive an email where they will be able to set their password.
After clicking save changes, the new user will be added into your organization.
The other way would be to access "My organization" from the menu:
First locate "My organization" from the menu on your left, and then select the "USERS" tab at the top.
After selecting the "USERS" tab, a new window will appear where you will have the option to "Add User" at the very top of the page.
Then, all you have to do is put down their important credentials, and if you want to set up their password you can, or they can wait for the email confirmation.
Don't forget to click save changes in order to add this user.
Below is a video explaining how to add a user to your booth