At this time, that feature is not available on the site. But thanks for the suggestion! When you're online, you're automatically visible in the location you selected.
If you are referring to chat, from the lobby, you can click on a room name to get a preview of who is in the room. While you are previewing the room, you will not appear in the room, but you can't interact with anyone in the room. Once you enter a chat room either by clicking the "Join" button or double-clicking the room name, your profile name and details will appear under the room name tab in the Guys Here window. Although you'll be visible, you don't have to join in the chat - but it's more fun if you do!