I also use the calendar feature on Yahoo and like it a lot.
I can enter family/friends birthdays to remain yearly, or even set up reminders for myself to do something six months from now.
You can set it to send you email reminders for events at predetermined set hours/days/weeks ahead of time.
I would think that to share it intimately, you would need to share password/screename.
However, there is also a function to email an "event" to someone.
I've also set up my whole addressbook on yahoo, so I can have my contacts available to me from anywhere I can get online. It's a bit tedious to set up, but useful.