How can I set up email for a small office so that all share the same address and see which msgs are unopened?

We are 5 people in a little office and all have the same email address, which works well for what we do. We use a webmail to read and write, but want to start using 'proper' email programs (preferably thunderbird), therefore we need some kind of system that will tell each of us whether somebody else has acted on a message. Currently we use the read/unread to show whether a message is pending or dealt with. Is there a way of doing this?

Trying to maintain a single email address through something like Thunderbird can be a pain. What you should be looking into is something like a ticketing system. A good one is Request Tracker. Your emails would be consumed by RT and you can keep progress on a ticket and know what's been done with it and if it's been resolved, etc..

If you really only wanted to maintain a single email account you could set up IMAP access to your email account and create subfolders by employee. They would transfer emails from the inbox and place them in their folders. When a task is complete they could then move the email to another folder called 'completed' or 'archive'.

I hope this at least gets you started with looking for options.