If you want to know who has been online recently, simply scroll down any forum page and click on the 'members' link in the statistics bar. This will bring you to a page in which you can see the recent (max one hour) activity of users on the forum.
If you don't want others to see your activity (only time is displayed and recorded, I don't want to know more
), click here
to see how to change your profile. (see 'Activity info')