If you want to see who is currently logged on to the system, go to the Users section and choose the View active user sessions action from the sidebar on the left. Note that this will only be available if you have permission to administer users.
The page will give you a list of who is currently online, what time they were last authorised to carry out an action and what time they last logged in. Note that users may have more than one session at a time, for example if they are logged on using a PC browser and a mobile browser at the same time.
Following the 'Last log on' link will let you view all the the times that they logged on as recorded in the system activity log, including the IP address that the logged on from.