I am on an Asus ROG Zephyrus G14 running Windows 10, this may not work for different computers.

1. Make sure all instances of Terraria.exe are allowed through your firewall / Temporarily disable your firewall I you're not sure, but be careful if you do disable it.
   
2. Use task manager to end all instances of Armoury Crate (there are multiple)
   
3. Go to "Services" in task manager and stop all instances of Armoury crate from running. If you don't do this, the program will probably reopen itself.
   
4. Use task manager to end "GameFirst VI" (there should only be one instance)
   
5. Double check that the 7777 TCP port is unoccupied by going to Task Manager > Performance > Resource Monitor > Network > Listening Ports. If you don't see Armoury Crate or another program using the 7777 port, you should be good to play with your friends.

I will add other people's step-by-step instructions for how to solve multiplayer issues here.

For the longest time, I always had trouble with getting the tModLoader multiplayer to work. I could occasionally get it working, but it was extremely unreliable. Often, I’d get into my friend’s world after a few tries, and suddenly be kicked off after some time for apparently no reason. Not only was this frustrating, but I didn’t even know why it was happening. I chocked it up to my unreliable internet connection, and the distance between my friend and I. We tried restarting the world, both of our Steam programs, and our internet connections, but nothing would solve it.

Now, after some testing and a lot of research, I can confidently say that the issue was other programs using the 7777 TCP port in windows. I imagine it like a USB port: only one connection at a time. Terraria and tModLoader use the 7777 port, as it is the default port for some reason, so if some other program is using it, no connection can be made. After figuring this out, and also using the troubleshooting guide (see bottom of guide for the link) to find out that Asus’ Armoury Crate program also uses the default port. I disabled that, and my firewall just to be sure, and it still didn’t work.

In a last-ditch effort to root out the problem, I tried disabling other Asus programs, starting with GameFirst VI. I probably got lucky, as the multiplayer worked immediately after ending the program. I can’t say that this will work for you, but if you do have an Asus computer, and the multiplayer isn’t working for you, it could work.

If you have a different computer issue that caused the multiplayer to be unreliable, laggy, or just not work, please comment it with how you fixed it so I can add it to the guide here.