So I was looking at a tutorial on how to play with friends in a private server but and my friends are still unable to join. How do I let my friends join my private server?
You have to forward ports in your router. Commonly you can reach your routers configuration at http://192.168.1.1 but it differs, your firewall might also be blocking connections. Ports that need forwarding seems to be:
(TCP and UDP 27015 should be the only required ones according to Valve: https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711)
Also enable the developer console and enter
sv_lan 0 when you have started your private game.