[WelshTom]

All passwords in a phpBB3 database are encrypted using an MD5 hash.

It's virtually impossible to decrypt MD5 hashes, but you should be able to find out their passwords before it is encrypted into MD5 format.

You could fiddle with the registration and login files on phpBB3 for this.

Anyway, you shouldn't be using phpBB3 on a live environment since its only a release candidate, and has unfixed errors.