Hello everyone! We've just released VRChat 2019.2.1, build number 746. You should restart Steam to get the latest update, and ensure you're not on any beta branches.
Until your friends update their version of VRChat, you will not be able to see them online. Let them know that a new version is available!
If you're a creator, make sure you grab the latest SDK, available at the VRChat Home website.