It's got improvements to group chats, improvements to voice chat, it has a two week long chat history you can keep, it has a new "persona" invisible mode, a new way to rename friends, a new way to trade, a new way to add friends, and more!
You can try this new functionality out in two different ways. You can either opt into the current Steam client beta by going to Steam > Settings > Account and opting into the currently available beta OR you can try it out via your web browser....