All encrypts do have a decrypt. It's a matter of knowing what the cypher is.
What you could do is send a link via email which will take them to a password change page as thought they had logged in normally (eg, set your password checker to "if 1=1" for that page, or something like that).
That or resetting their pass, and sending an autogen pass to them by email. Both have the problems with people plucking them out of cyberspace, but the top one would probably be slightly more secure, as someone interested in random mayhem is less likely to bother following a link and entering new pass and everything, than just using a pass... the difference is marginal however.
A better way that plain text would be to send them the password in a .txt file which you had changed the extension of (to something like .dud) and then instruct them to change the extension back to .txt. This is a lot less likely to be picked up by a password sniffer.