Justcloud makes it super easy to share your folders.
First login to your online control panel and locate the file or folder you want to share. Once you have found it hover over it with your mouse and an option will appear called 'Share'
Clicking Share will take you to a page where you can enter your friends email address and a personal message. Upon inviting them to share the file or folder they receive an email inviting them to either set up an account or with a link to login and accept the share if they already have a Justcloud account. Last Update: 02/20/2013
Justcloud is currently working on bringing this feature to it's customers.
Currently within the Control Panel there are options to either share entire folders or single files. Last Update: 02/20/2013
Unfortunately Justcloud can not re-invite you to join the folder. You will have to contact the owner of the folder and request a new invitation to be sent. Last Update: 02/20/2013