What is the best way of sharing a Calendar with a group? Google Calendar, Fruux and KolabNow, for example, are no good as they do not fully support standards and it is not possible to work with tasks (VTODO) whilst in the calendar window.

KOrganizer does this brilliantly (as does T'Bird/Lightning) but I need to share with other members of a groups who may be using different OS platforms.

Can anybody help please?