See: https://playnightingale.com/news/play-with-friends
For multiplayer, you can use the host/join mechanics where one person is the host and everyone connects to that host. The host will need to open ports in their router and set up port forwarding. How to do this is highly dependent on your individual router model and ISP, and is not something that we can walk you through very easily - but check your router configuration page for options like 'NAT/Gaming' 'Port Forwarding' or 'External Connections'. The host's public IP will be required by everyone connecting.
You can find your public IP by visiting a site such as whatismyip.com
If everyone connecting to you is within your home network (that is, living in the same house as you), you can use your private IP instead, no port forwarding required - like LAN parties of old. See https://discord.com/channels/854679517997695016/1423434949272408255 for more information on host/join mode.
You can also download the dedicated server architecture from steam for free (even if you bought the game on epic) and set that up. Everyone will then join the IP of that sever - you'll also need the public IP & port forwarding setup for that. Setup FAQs can be found: https://discord.com/channels/854679517997695016/1395472159568040017
Alternatively, you can rent a server from GPortal. No setup required on your end, just use the IP and port that GPortal gives you. This is the only option that costs money.
-# Thanks Koriania for putting this together!