OpenERP is essentially stateless. Every request made from the web client is logging a user in. Every user can have the web client open (and open in many browsers, in many tabs and even on many machines), but there is no load on the system if they are not doing anything. If they spend 5% of their time clicking/typing and 95% of their time reading, there is a very low server load.

So you can't get the users who are logged in, only those who have recently connected. At version 7.0 this is even harder unless you log connections because the table res_users (model res.user) that stores the last connection date for each user now stores only the day, not the time (like it did at 6.1).

What do you want to know the users who have recently logged in for? To check that you can reboot the system? To model/manage server load? Perhaps other readers of your question may have some ideas if you explain the need behind your question.