If you want to play coop or Versus matches with friends the player who hosts te session will be the session host. This means that the game server will run on his pc. It is recommended that the player Hosting the game will have a fast processor as his computer will do extra load to host the game server.
As Saranity is a Unreal Engine based game it will use the following ports for network communications with connected clients. If you are a client in most cases there is no need for changing any firewalls or the windows firewall. If the Windows Firewall ask for permission you should let the Saranity.exe allow thru the firewall and it will configure the app permissions.
If you need to configure any firewall yourself the following should be taken into account:
As Client:
Tcp port 443 outbound – Communication with Master Server to retrieve sessions
Tcp port * outbound – Connection with the host server to tcp port 7777
As server: (Hosting a game)
Tcp port 443 Outbound – Communication with Master Server to update sessions
Tcp port 7777 Inbound – Inbound connections from clients to the game host
Whenever you host a game session to allow the clients to connect you will most probably need to add a port forward to you router. Because your computer network is not using a public ip address but your router does, you will need to tell you router where your server host is located so it can route clients to the correct IP.
How you do this is dependent on you router make and model. Most models and brands have the explained in your router manual and most of the time you can find this under a chapter called port forwarding.
Make sure you forward port TCP 7777 to the private IP address of your computer.
Multiple guides for a lot of brands:
https://portforward.com/router.htm
https://www.noip.com/support/knowledgebase/general-port-forwarding-guide/
For more general information see:
https://en.wikipedia.org/wiki/Port_forwarding