SmarterMail doesn't contain any login reporting at the moment. In the Administrative Logs (set to Detailed), you can see webmail login attempts ("Login successful: With user email@example.com."), and in the User Activity section, you can review the current Online User connections and the last login date of Inactive Users as well. However, there's not a trend report within SmarterMail to display this information for each user.
That said, I'll go ahead and change this thread from a Question to an Idea, and we can facilitate tracking on the request of adding a system login report for users.