Another features you may consider

Add loading indicators

When frontend communicates with the backend, it's good idea to show user that something is happening.

If you want some cool loading animations (aka throbbers), check out this site for their CSS & HTML code ready for copy&paste.

Poll server for participants list updates

• use setInterval to query the server periodically for new data
• verify behaviour of your app in two browser tabs - when you add participant in one tab - it should be visible almost immediately in the second