Hi!
First you must portforward your computer on port 7777.
After that is done give your friends your IP address, Google what is my IP. Do not give your IP address to people you don't know.
They need to delete 127.0.0.1 from the join IP box by pressing backspace and type in your IP address.
Once that is done they must press enter.
Hope this helps, cheers!
Ammending this post to say that the game now uses lobbies so that players don't need to port forward or anything just enter the host's join code.