NOTE: This answer applies to legacy accounts (i.e., those that previously existed, including AppSumo-linked accounts) as they were grandfathered into the current system and remain free to access until such accounts are closed or deleted by the account owner.
In 2018, when we removed Teams (it was an underutilized feature and added an unnecessary layer of complexity to all aspects of Airstory for our users), added free collaboration with guests and lifted the cap on projects, legacy accounts gained access to unlimited projects and unlimited collaborators for the life of their account.
Although Collaboration is still an important part of Airstory and you can still invite guests to your projects, if the guest you are inviting is not an Airstory user, they can use a free temporary account by clicking on the Go to document button on the login page.
Temporary accounts are free to use and they are full featured but they expire after 5 days. There is no limitation on the number of times a person can use a temporary account so if you need them to collaborate with you several times over the life of a project, you can send them a shareable link which will allow them to return to the same project time and again.
If your guests are going to use Airstory regularly or for more than collaboration on your projects, the best experience would be for them to create an account in Airstory in order that they can keep track of all the projects they have been invited to or they can create their own.