To view topic members click on the person icon located at the right-hand side of the topic. Darkened profile images indicate the user is offline.
To view users who are online, click the "Person" Icon at the right-hand side of the Topic name. Clicking here will display all of the Topic members' icons and usernames. If the circle at the bottom right of their icons is green, those members are currently online. If it is grayed out, then they are offline.