To make an existing folder a Collaboration Folder, click on the little box with the downward arrow and select 'Invite Collaborators' from the 'Share' sub menu. A pop-up window will appear. Enter the email addresses of the individuals you would like to collaborate with.
NOTE: This must be the same email address your collaborators have used to register for their Box accounts. Please verify this before you proceed. If a user does not yet have a Box.net account, they will recieve an email inviting them to sign up for a free account. Once they sign-up they will have immediate access to the collaboration folder.
Once you have entered them all, click 'Ok,' and email notifications will be sent to those contacts. Once they log into their Box.net account, they will be able to accept or reject your invitation to collaborate on that folder.
When a folder becomes a Collaboration Folder, its icon will change: Rather than being a plain, yellow folder, it becomes a blue folder with a small image of two people on it. All collaboration folders will automatically appear at the top of your files list within your account.