Excel: Find the most common value in array given a certain value (IF, Index, Mode)

I'm trying to find the most common value in a range given a previuous value occurs, so for instance, if Shared mailbox (Col C) is equal to Finance tell me the most common team number (Col B) of the users who access it:

The function I have so far is (obviously ignore the absolute cell references)

=INDEX($B$2:$B$20,MODE(MATCH($C$2:$C$20,$C$2:$C$20,0)))

I could do this with a pivot but I need to insert the value into a cell and vlookup in a pivot wont cut it. Also there are tens of thousands of rows. But essentially the pivot result is what I'm looking to get to, just in a cell via a formula.

Here is an example of the table, a pivot and a peek at the formula and results.

Excel image with pivot

Any and all help is welcome. Cheers Matt