I think it depends on the situation. If it's a public event and you happen to be someone in the crowd then you would have no control. That is, after all, what the newspapers do all the time. If it is a closed group such as an event at a school or club, the organisers should set a policy on photographs, taking into account any individual objections, and make it clear to all those attending. If it is a private event then, absolutely, nothing should be posted without express permission.

DC