Yes...
I've had a .Mac account since last October and I purchased an email only account for my fiancee in June. I was only charged for three months and was told that it would renew when I renew my membership. If I don't renew my membership then hers will expire as well.
As for changing the the password, I don't believe so. You can set it and have access, but if the user switches the password themselves and you don't know what it is then you're s.o.l.