The meeting started before all the sales staff ................ (arrive).

There are two options given in the key to this exercise: had arrived/arrived.

The past perfect is used here both to show the order of occurrence of two past events, and to indicate that all the sales staff did, indeed, arrive subsequently. The sentence is constructed in a way that puts the emphasis on the organisation of the meeting, which was not delayed to wait for the sales staff. This construction would be used e.g. if one wanted to criticise the organisers of the meeting for not waiting.

You suggest an alternative: "The meeting had started before all the [sales] staff arrived." This puts the emphasis on the staff who missed the beginning of the meeting and would be used e.g. if the sales staff were being criticised for their lateness.