If you are working as an employee at an organization, when you tell about it to others which usage would be more appropriate?
- I work for company.
- I work at company.
Or are both usages correct?
Both usages are correct, and they mean the same thing.
The only minor wrinkle is that at can be used to refer to a physical location as well as a company itself:
Whereas for cannot be used this way:
The last sentence would somehow indicate that you are employed by the campus itself, which probably isn't what you mean.
You work for a company at a location.