I am being asked for a tool that will allow specific users to view and edit a online calendar. In my office there is a need for 10 users to be able to all share an online calendar for office tasks / communication. They don't want the calendar to be visible by everyone on the LAN with access to a HTML browser. Does anyone know a tool that will work as such? They just all want to share a calendar and be able to schedule events and such between themselves.

I don't know if this matters but the company web server is CentOS Linux and the clients (10 users) who need to edit this all use Windows XP / Mozilla Thunderbird.

Have a look at Davical. It requires PHP version 5 and a PostgreSQL database. I haven't used it personally but it looks good on paper and should work well with your Thunderbird clients.

http://www.davical.org/