。
I'm at the office because I'm busy.
Literal
Busy-[て] company [at-に] am [explanatory-んだ].
The te-form 忙しくて gives the reason for being at the office. んだ (= のだ) adds explanatory force — the speaker is accounting for their situation, not just reporting it. に marks the company as the location of existence with いる. The casual tone and んだ make this feel like a text message or phone call to someone close.