Organization domains are used to provide automatic access to certain users. More specifically, here you assign domain names to your list of approved domains. Users who log in with an email address that matches one of the added domains will receive access to your organisation.
As can be seen in the image above, the domain "vodafone.com," can be added to the approved domains list by clicking + Add Domain , filling in the domain on the right side "vodafone.com" and hitting Add.
This means, that every user that uses the email with domain "vodafone.com" will join my company.
(i.e. James@vodafone.com, Emma@vodafone.com, Chris@vodafone.com)
NOTE: Beware to use general domains such as @gmail.com, @hotmail.com, etc. this will enable an enormous number of users to access your company's desk booking.