In that case you should panic...
No, seriously. The password can NOT be retrieved.
It's encrypted, and stored in the db with an algorithm (Secure Hash Algorithm 256) which is a so called one-way encryption algorithm. E.g. the same input provides the same output, but the output cannot be used to reconstruct the input.
Check
WikiPedia for more info on that algorithm.
Regarding your question ; if your users do not have a network login, how are they using the computers ? Are they using no login at all ? Just 'guests' ?
If they're not using any secure login on your network/PC's, why should they be using a password on the forum then ? Just set them all to "welcome" and your done....
It seems a bit strange to me to let users on your network and intranet without any form of identity management, and expect the forum to work around that issue, but that's just me.
Look, security is as strong as its weakest part, and with NO security on the PC's used (e.g. open access to the PC's, with no network login), the cookies used by the forum are accessible to anyone with physical access to those PC's.
Access to the cookie means access to the forum...simple as that.
So, if that's the current state of the environment on your network, why bother with any form of security.<