EDIT: When the server is “full”, this will not work also. So, forget about it, move along.
For me at least, the ingame browser does not work when it comes to joining my friend’s server(I can’t even see any servers there).
But if you alt tab out of the game, and use steam server browser’s “friend tab” to join server it will work.
Hope this helps somebody until devs fix the server browser issues.
Shift+Tab not working for you?
Sometimes it does not, I get some error message.
This way you can also check for server info. It is just another way of joining your friend’s game, I guess…