I assume you mean you have created the LAN server yourself and your friends are trying to connect to it?
Usually when people can't see your LAN server, it's due to port issues and/or strict firewall and/or antivirus settings. Can be from both ends.
Back when I was hosting a local server, I had to forward these ports in my router: TCP 27014-27050 and UDP 1200, 3478, 4379-4380, 27000-27030.
Have in mind this was Counter-Strike: Source, and the other Counter-Strikes need other ports to function. I'll list them for you below.
CS 1.6TCP 27020-27039 and UDP 1200, 27000-27015
CS:GOTCP 27014-27050 UDP 3478, 4379-4380, 27000-27030
Feel free to PM me if you need help setting it up.
