Can I find out which ssh key was used to access an account?

Is it possible to find out which ssh key was used to access an account? I have an account on a server that I let several (trusted!) people have access to via ssh. I'd find it useful to be able to know who logged in and when. I have root access so I can look at the logs, but there doesn't seem to be anything there. Is there some configuration switch that will put some way of identifying the key in the logs?