Thank you for your post.
Make sure that the file is set up for network sharing. Pull down the File menu and select "Sharing -> FileMaker Network...". Turn on Network sharing. Once turned on, the bottom left area of the dialog box displays the currently open files. Select the file you want to share, and then on the bottom left side, set the network access to all users.
Now, when the next user select "Open Remote..." from the File menu, they should be able to open and share the file successfully.
In addition to what TSGal mentions about sharing the database....
UNDERSTAND that you need to SHARE the file using FMP or FMS.
You can't just have the filename.fp7 file on a shared directory and have multiple persons access the file...that won't work. It must be shared/hosted with either FMP or FMServer.
You need to be aware that allowing access in both ways puts the file at (high) risk of (catastrophic) damage. Best practice is to only allow access via FileMaker network sharing; hence removing other access; which is best done by putting FileMaker on its own machine; but with 10 people you'd be (much) better off using FileMaker Server. Yes, more expense, but how much is your data worth? Not to mention (much) improved access speed, multi-threading, automated backups, etc., available only with FileMaker Server.