How do I enable users access exchange2010 using OWA?

There is a problem being currently being experienced by other users whom we share a public calendar in Exchange 2010. I happen to have all the owner rights to this calendar. The other user’s access their Exchange accounts via OWA. However the users are unable to set a reminder since the check-box for this purpose is disabled and after they complained I went to correct this situation but I was shocked to find the check–box grayed although I managed to add a new event and check the reminder box which was functional as I confirmed it later when I went into my outlook. The other users have no problem creating and editing their events /appointments in the calendar. My question is how do I enable these users accessing the exchange accounts via OWA to set reminders if possible?
