modification needed: changing all passwords
 

Hello
i have a problem that for some reason some one manage to get the passwords for some members we don't know how many, and he use them to post illegal posts

what i am asking is some thing like this

command to change all the passwords for all users putting in mind group id, also when user is logging in some how it will show him how to retrive passwords

or
when ever user logged in with his passwords it will reset his current password and will send it to his registered email, so he can retrive it from his email, that will be fine and this will be better
any one can help in this

thanks

any one can help in this ????