Every big forum software tracks online users and guests and it can display each user online status, as well as online status overview according to user roles. And now, with GD bbPress Toolbox Pro 4.0, bbPress forum can do that too.
Plugins Tracking settings panel now has new group of options to control how the online tracking is set up. This includes tracking of users (logged in) and guests, and the online period interval used to determine users last activity, default is 5 minutes (or 300 seconds).
For front end, plugin adds a new widget called Online Users that will display a summary of the number of online users and guests, and the list of users by each role. This widget uses template for display, so you can copy the template to your theme and customize the way it looks.
Also, you can display online status for every user in the topic or reply user area, and that can be enabled from the Topics and Replies Settings panel in the group of settings for display of user statistics.
Online tracking is not 100% precise, it depends on the online period, and you can use smaller value (300 seconds is default), to have smaller window for user counting. If the WordPress heartbeat system is active, online tracking precision will be higher.