Dropbox rolled out a small but important update to the Share Folders feature. Now, users are able to control who gets invited by others to the folder, albeit rather crudely.
The original creator can decide whether others will be allowed to invite more people to join or not.
When creating new share folders, you'll see the option as a check mark labeled "Allow members to invite other people." The same option is available for existing shared folders.
It's a welcomed addition, but users are now asking for more. Specifically, the ability to share folders as read-only is highly requested. Another "nice to have" option is the ability to allow others to contribute, i.e. add new files, but not modify the existing ones.
Of course, a big part of Dropbox's success has been its simplicity, so any new feature or option just adds to the complexity.