Dropbox does not offer users the means to directly send files to one another. However, it allows them to share files through the Dropbox website or via the Dropbox application.
The procedure for sharing files via the Dropbox website is the same across all devices. Simply sign into the website, go to your folders, and hover the cursor over the folder you want to share. Click on Invite People to Collaborate, enter the email addresses of the users whose access to the selected folder you wish to grant, and click Share Folder. The shared folder appears in each user's Dropbox once they accept the invitation.
To share a folder through the Dropbox application on a computer running Windows 7 or 8, open the Dropbox folder, right-click on the folder you want to share, and click on Share This Folder. Using the window that pops up, choose the people whose access to the files you want to allow.
If using a Mac that also contains the Dropbox application, follow the same sharing procedure. However, instead of waiting for a pop-up window, wait for the Sharing page to open on the Dropbox website, and follow the necessary procedures for sharing through the site.