Official comment
First of all, when sharing the folder the recipients must be granted Editor rights if any updates are to be made by them.
When you share a folder with recipients, they will automatically get a Syncplicity account associated with their email if one does not already exist. In order to update content in the folder they need to install the Syncplicity client which will automatically sync their updates to the folder owner and other recipients. Alternatively the recipients can login to my.syncplicity.com and upload the changes to the folder there, but this will not happen automatcally as it does when using the client.
There are lots of resources on our site that show examples of folder sharing options and actions to take.