This guide will help you with the basic old way of joining friends either that being your first time joining them or if the social servers go down and you have no other way of finding them.
Step #1: Finding out what server they are in
Say your friend is in a map they have no clue what the name is and all they can do is describe what it looks like, the first thing you will tell them to do is to push "esc" than tell them to click on "worlds" that ask them for the name and number e.g #25.
Step #2: Finding their server
Now that they have given you the info you're going to find the map name they said either in search or the tabs e.g popular, active, once you have found the map your going to want to hold click and drag the various hubs to the left while also looking for the number they said e.g #25.
All finished and ready to go
You have now found the server and should be good to go, if the server is full you will need to create a new instance than click on public (optional if they are already in your friends list) and repeat the steps above again after they have joined their new server room.
I hope this guide was a big help and gave you a secondary way to join friends, enjoy making new friends in vrchat!