This feature does not yet exist because MantisBT doesn't maintain a list of sessions. Instead MantisBT relies upon PHP's built in session management.

You can't officially enumerate PHP sessions unless you start using hacks/workarounds. See http://stackoverflow.com/questions/675913/looping-through-all-a-servers-sessions-in-php for an example which only works in specific circumstances.

For what you're wanting to do it'd be best to create a new database table within MantisBT that tracks the "last access time" for each user so you can determine approximately how many people were connected within a certain time period.

There is no need for a new database table.
The existing user_table has a column last_visit for this.
There is also a prepared but not used function user_get_logged_in_user_ids in core/user_api.php to get an array of currently estimated logged in users

Thanks atrol, good catch:)

Thank you very much