You can sit on various chairs in the 3D world. If you are standing, simply mouse-over a chair and a highlighter will appear around it. Click the chair and you will transition to sitting.
To stand when sitting, you’ll notice a blue button that says <STAND UP> on the right of your screen. Click that to stand. Alternatively you can use a hotkey: the default hotkey to stand up is the Page Up key.
You cannot sit on an occupied chair, and you must stand before sitting in another chair. A chair may not register as ‘sittable’ if you are standing too close to it.