CentreStack allows you to share your files with any external email address, not just with licensed users. You can use the web portal, the Desktop Clients, and the mobile apps to achieve this.
If you require your share recipients to be able to edit the original shared files, then the process of sharing with full-permissions will automatically create Guest Users in CentreStack. Each Tenant is allowed to have an average of 3 guest users for each licensed CentreStack user. Sharing with full-permissions will cease to work once this limit is exceeded, but you can delete guest users once they are no longer needed to recycle the slots. This guest user quota is enforced only at the tenant level, so it would be OK with 2 licensed CentreStack users to have user A create 5 guest users, and user B create only 1 guest user. For more information about guest users, please check the "What is a Guest User?" article.
To invalidate full-permission shares, the tenant administrator can delete the shares from the File Browser, Tenant Reports->Shared Objects page, or the Tenant Dashboard ->Guest Users page. Please note, that deleting the shared objects from the reports or file browser does not remove the guest users themselves. Likewise, deleting the guest users from the tenant dashboard does not delete the shared objects. To prevent ending up with broken shares or guest users without any content, remove both the guest users and their shared objects once they are no longer needed.
You can also disable the ability to create guest users from the Tenant Dashboard->Access Control->Allow Creation of Guest Users setting. This setting applies to all users under a tenant. If you need to disable the guest user creation ability for only a few entities, you can create a Role from the Tenant Dashboard, and apply it to only one or more users or groups. Under the Policies tab of this Role, be sure to deselect the Allow Creating Guest Users checkbox.
If you only need to share folders or files with read-only and/or download permissions, then you can do so for an unlimited number of email addresses. Alternatively, you can simply create "public links" to folder and files, so anybody can access those items without any authentication. Public links can only be read-only.
Both read-only sharing and public links do not create guest users and can be shared with an unlimited number of external email addresses. If you want to stop sharing, you will need to delete the share from the file browser. Once a share is created, a shortcut to the share will be created under the Shared Files/Folder->Files/Folders Shared By Me section. You can use this shortcut, or go to the All My Files page and open the Share popup to edit the share details.