At present it is easy to make a Group or Lookup of Contacts Private. Unfortunately the same can't be said for Activities.
The problem is that, although Activities for Private contacts don't show the contact's name in the calendar, it does still show the subject. So if you you've suddenly opened your database to another user and you want to hide the details of all your personal Contacts along with any associated activities then you will have to open every single activity individually and mark it as Private. OK if you've only got a few but a massive administrative burden if you've got a few hundred.
Can we please include the same functionality we currently have for Contacts into Activities.