The On-line users module can be enabled in Settings -> Security & Membership by selecting the Monitor on-line users check-box.
If you are running the system on a web farm, also select the Store on-line users in database check-box. This causes information about on-line users to be saved in the database and also allows the system to provide more detailed information about guests (anonymous users).
The Update on-line 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, enter the same value which is set for the Remove expired sessions scheduled task (you can read the value in Scheduled tasks -> edit Remove expired sessions task -> Task interval -> Every: X minute).
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.