If you leave a shared folder, you can re-join it via the Manage Shared Folders page of your Web Admin Panel. Simply click Rejoin next to the desired folder under the 'Left folders' section, as shown in the image below.
Once you click 'Rejoin', the folder will appear under the 'Manage shared folders' section as well as inside your AeroFS folder as an empty folder. The contents of the folder will get synced over from another device that contains the files. It is important to note that at least one other device must have the contents of the folder you are rejoining, in order to successfully restore the folder. In the event that all members have left the shared folder, you will only be able to restore the folder if there is a team server that has a backup of the folder.
The 'Rejoin' option will only be available if you left the folder on your own. If the 'Owner' of the shared folder removed you from the shared folder, you will not be able to rejoin the folder on your own. In this case, you will have to request the 'Owner' of the shared folder to re-invite you to the folder.