Using signing in a group environment

[Brian Rectanus]

I would like to create a secret key for our group here at work so that
we can send out signed email.  What would be the best way of doing this
so that when a group member leaves, they would not be able to continue
to send signed email?  If a member should leave, would the secret key
have to be expired or is there a better way?  If the signing key was
expired, how would this effect any older signed messages?

Thanks,
-Brian