Learn Farsi: ما در یک دفتر مشترک کار می‌کنیم.

Edare usually refers to a larger administrative department or government office. Daftar is used for a smaller workspace, a private office (like a lawyer's or real estate agent's), or the physical room you work in. It also means notebook!

Does یک (yek) require the indefinite -i suffix on the noun here?

In standard conversational Farsi, you just use yek followed by the noun and an Ezāfe to connect the adjective: yek daftar-e moshtarak. Adding the -i suffix (like yek daftari-e moshtarak) sounds very formal or literary.

The word مشترک (moshtarak) looks related to other business words. Is it?

Yes! It shares the same Arabic root (sh-r-k, meaning sharing or partnership) as words you likely already know, such as sherkat (company) and sharik (partner). Grouping these root words makes them much easier to remember.

Since we are learning about magar inke (unless), how would I change this sentence to say 'unless we work in a shared office'?

You would add magar inke at the beginning and change the present indicative verb kar mikonim to the present subjunctive kar konim. The whole phrase would be: magar inke dar yek daftar-e moshtarak kar konim.