Shared Workbook error: File is locked.
 

I used the Tools/Share workbook option for a file on my company's share
drive. I can open the file just fine, however, when anyone else tries to
open it, they get the following message:

File is locked. In order to save changes to the file, you can either close
the file before making any changes and then re-open it, use Save As (File
menu) to save the file using another filename, or turn off Shared Workbooks
and then save the file.

Why are they getting this message?