You can get a report of either all, or selected Users and their log-in history. In order to report on this information you need to carry out this step first:
Go to Additional Settings > Audit Log Settings and enable the following: Log User Logins into the System Audit Log for reporting
Note that any user log-ins will not be logged in the system audit log prior to this being enabled.
To create your report:
Select Reports from the Jump To menu on the left panel and Create New Report
In the menu select the Audit Trail option in the top menu and click Continue
The columns that will be relevant in the report are User, Action and Date:
Choose the date parameters you want to report on, and set the "Action" to contain "login"
You will then have a report showing you all your user logins:
To show you a total amount of logins each user has made in that time period, you can also Group your report by Users.
Set the Report Type to Summary - this will display the Grouping options.
Set the Groupings as follows: