Ok, Paul, So I took a few minutes to figure this out (assuming this is where you’re going with this):
Here’s the MySQL query that will give you the latest post date, one user at a time:
SELECT post_date FROM wp_posts WHERE post_author = 1066 ORDER BY post_date DESC LIMIT 1
(The number “1066” is arbitrary and represents a specific User ID on the table that I was testing this. So you would replace this with a given user ID for the user that you want to determine their latest post submission. )
So this simple bit of MySQL you would use in a database manager (like PHPmyAdmin) on the SQL command screen. A better solution would be to create an admin page that you can access from the WordPress Admin that can then list all your users and their latest contributions side-by-side. But that query is the heart of what you’re after, I think.