password problem

They might be signing in on the wrong side of the login screen. On one half is the Invite Key sign-in, which requires the key that you set for the wiki, and on the other half is to sign in with a PBwiki Account, which can be gotten at my.pbwiki.com.

If you want to make things simple, you can have them sign up for Accounts, and then give them access by adding their email addresses under [Settings|Users] and then they don't have to worry about the invite key at all.