Firewall - Possible Issue #1
As for a lot of games, you are given a Nat type such as open, restrictive, etc. If your firewall is not allowing any connections this would be why your friends are unable to join. Open up you firewall and navigate down to Inbound rules on the left side. You should now see a list of applications on your computer in which you want to scroll down until you see The Escape. When you see The Escape right click on it and go to proprieties and see if it is being allowed. It should show allow, if not switch it to allow and try to have friend join. If you'r friend still can not join move down to the other step.
UPnP - Possible Issue #2
Our networking tries its best to automatically port forward the ports for you to allow the other players to connect to you. This is done by using UPnP which is an option for every router which can get disabled. If this is disabled the game will not be able to automatically port forward for you which will result in no one able to join your game. If you do not want to follow the steps to turn this on or this doesn't help you please move down to the last fix. To check if this is turned on you will have to go to your routers website by opening up your web browser and typing in 192.168.1.1 which will then prompt you to log in. Once you logged in you are going to want to find where your UPnP is located mine being under Advanced > Advanced Setup > UPnP in which mine was switched to on. If you can not find this don't worry you can just follow the last fix which will solve all issues for most multiplayer games.
Port Forwarding - Possible Issue #3
As said above this will 100% solve the issue you are having and this will never have to be done again unless you IP changes. As said above you are going to want to open a browser up and navigate to your routers default gateway by typing in 192.168.1.1 which will then prompt you to log in. Once you are logged in you want to navigate you your port forwarding section and open up port 7777 for both TCP/UDP. Every router is different but it should be a very simple process and should solve you issue with your friends not being able to join.