The On-line users module can be enabled in Site Manager -> Settings -> Membership by checking the Monitor online users check-box.
If you are running the system on a web farm, you also have to check the Store online users in database check-box. This causes information about on-line users to be saved in the database.
The Deny login interval property determines how long users will not be able to log-in after they are kicked. The value is entered in minutes.
Finally, the Update online users (minutes) property defines how often information about users accessing the site is reloaded. The value is entered in minutes. When running the system on a web farm, you need to enter the same value which is set for the Remove expired sessions scheduled task (you can read the value in Site Manager -> Administration -> Scheduled tasks -> edit () Remove expired sessions -> Task interval -> Every: X minute).