Nakano15
Terrarian
If I remember well, to turn a project private on GitHub I'll have to pay, what I don't plan on doing. So I don't use it.Oh, you should have a look at git even if you don't plan to make it publicly accessible; the ability to do proper and reviewable source control is an invaluable tool for developers. (It's completely free, too. You can look at either Github or Bitbucket for starters, just make sure you set up the git to not be public by default for the points below)
Anyway, you do have a point of forum rules and releasing Terraria source which I had not considered, I guess that could be a problem in making it public...
but that depends how the mod is built, do you only interface the Terraria code to feed in your own code, which is in separately handled scripts - or is everything compiled in the same package and you edited the T files directly?
If it's the latter, then it'll be difficult to make it open for contribution... if you went through the trouble to create interfaces and your code is decapsulated from it, then it should be quite easy.
Also, on the mod I place mod method links to Terraria codes on their respective places, leading to mod codes (or at least most of them, since a part of the mod codes are still on Terraria codes), that could turn easier to edit and share the mod source without sharing the source of Terraria, and not violating TCF rules, but the ones wanting to do changes on the mod would still need to decompile and fix the bugs the decompilation if they want to test the changes on the mod.