How can I see who is logged in?
This can be found by clicking into that little "flag" icon (also known as the 'Notification Center') located in the upper-right portion of the page - just above the menu bar.
Once you prompt that drop-down menu you'll find that the last list item shows the number of members currently online.
From there, if you're curious to know exactly who those people are, you can select the button for 'view members' and you'll be provided with a list of names.
If you see a list of members, they can also see your name. If all you see is "Sign in to view list", nobody can see your name - though you can change this instantly by clicking that sign-in link and then clicking the 'submit changes' button at the bottom of the settings page (which you will be automatically redirected to).
If you wish to remain anonymous while logged in you will need to select 'change settings' from the drop-down menu spawned under you user name, navigate into the 'account settings' tab, and check the box that says, "Hide my visits and online status".
The status for online members is updated every 10 minutes. A member may therefore appear online although they left during the last nine minutes. It's also possible for someone to have been using the same page for ten minutes and thus appear to be offline. To that extent, this is not always a completely reliable list.