If your goal is to share email having offline folders does nothing for that. Remote folders are what other users can access, if its either a shared account (what you seem to be using) or configured as a "public folder" (requires admin to set ACL's to let people access a folder that is not in their account).
Offline folders are a poor backup as they can get automatically wiped when you start Thunderbird if you sync with trashed remote folders. If you want a local backup I'd suggest disabling offline folders, and use a message filter to automatically copy your personal inbox and sent folders contents to folders in Local Folders. Or use something like IMAPSize which can do automatic incremental backups to .eml files. By incremental I mean it doesn't backup a message again if its already backed up. It can't be configured to run as a windows service but it supports command line arguments so you could do something like change your TB shortcut to point to a 2 line batch file. The first line runs Thunderbird, the second runs IMAPSize.
http://kb.mozillazine.org/IMAP_backup