I see that I can share as long as I create the calendar on the net, however it seems as if I would be the only user able to modify the shared calendar. We want to use this for business purposes as we like the features so far, but if the creator is the only one able to make changes, I will need to keep looking

I guess it depends on how you created your calendar 'on the net'. Some years ago I achieved something like you want by creating a Google calendar, and when I was in work before that I used Yahoo. Both allowed me to set up a calendar which could be shared with others, and both allowed varying privileges to the other users -- e.g. some people could just view the calendar, others could add/edit their own entries, others could add/edit all as far as I remember. I don't know whether those facilities still exist, and since you don't say who is the provider of your calendar on the net I can't advise further.

I used Teamup to create an icalendar to use with Thunderbird. It seems in Teamup i can grant different permissions, however once it is integrated to thunderbird, I do not have the same capabilities. I appreciate your response, I will check out yahoo to see if they still offer those permissions

One approach would be to set u a new account for the sole purpose of the shared calendar, then tell all users its login and passwords. You would all collectively be the "owner" of the calendar so you'd all have full permissions on it.

I'm thinking how I'd do it using Google Calendar, or the similar caldav-enabled offering from gmx. I don't know Teamup so I have no idea if this would be workable.

Thank you for your response, the problem with that is we are unable to track who makes the "changes" or the additions to the calendar as well as the fact my boss does not like google's calendar or outlook.

Does anyone know of another desktop calendar with email integration that allows multiple administrators aside from the above mentioned?

Your asking the wrong question, and like so many folk, your not finding the answers you want.

The correct question is does anyone know of a calendar server that shares calendars in read write?.

The answer to that is yes, many of them Google, Yahoo, GMX,

On a more personal level you might want to look here http://caldav.calconnect.org/implementations.html

Caldav is open standard that calendars use on the web, that site among other things lists caldav server implementations and clients. Noting that Thunderbird with lightning is a "client"

If you run your own Linux server, SOGo might be a good fit. http://sogo.nu/download.html#/zeg