Anything programming-related, whether involving games, game mods, or computer software in general, can be posted in General Programming.
If it's game or mod specific (and doesn't apply to general programming principles), it can go in Game Programming. If you are new to programming, it can go in For Beginners.
Anything art related, whether involving games or not, can go in Visual Arts.
Anything game design related, whether involving new games, mods, or just pure concepts, can go in Game Design.
For recruitment for mods, games, or other projects, we have the Classified section.
As far as I know, we don't have a heavy modding community here, but we have a generally all-around technically-savy community here, working on all sorts of projects that usually (but not always) involve games. This includes a few mod projects that have recruited here in the past. For specific details involving a specific game engine that you're modding, we might not know the information off the top of our head, and it might be better to seek the community that's developed around the original game, but for anything else, this community is very friendly and knowledgeable as long as those coming for help first put in effort themselves before asking for answers.