How to get a ip address?

This might not be the answer you were waiting for; What is the point? You can't hack them with an IP adress unless there is an OS/Software vuln, can you?

Patched, still works in TF2 though but, if you have Admin status or higher you can see a player's IP when they join or if you have RCON.

EDIT: To see their IP when they join have developer 1 on the client [requires Admin [or higher rank] or RCON]

Use a steam username ip locator. Or rcon

If you're an admin then make a server-side script that prints it to you.
Player/IPAddress - Garry's Mod

You can use something called wireshark, all you need is the person of your friends list skype or steam. Then you open wireshark type classicstun (up in the box) do not hit enter, then call the person steam or skype and hit start give it about 5/6 rings and end it. Then hit enter on classicstun. After that you will see a bunch of numbers. There will be a number that was 28 in front of it. And you will have their IP.

uh if he has their then he could just use a skype IP resolver

But if he does not have their skype he could not use a SKYPE ip resolver. And also i was just mentioning other things you can use Wireshark with.