For a faster answer, use the QUESTION tab in the upper middle if you want it faster, and how they join it
?
Open the console, see the IP adress, post it somewhere, make your friends copy it (And the port if you use any) in the mulitplayer thingie, they should now connect.
If you're using localhost it wont quite work the same way but, they have to get into your network and then can join you.