If you're passing a dollar bill, it would be 5 people. 0.1*1x=0.5; x=5

If you only have $0.90 after the first pass, because you took out the taxes, then it is more complicated.  I don't think there will be a good formula because you have to round to the nearest cent, and rounding error will become very significant.

Tax - left
10c - 90c
9c - 81c
8c - 73c
7c - 66c
7c - 59c
6c - 53c
5c - 48c

More than half gone after 7 people. 

 Ehhh, I always round the final number only, so I'd just let the formula proceed as it wanted. But jfc, even just doing the hand math; 7 people and 52% gone 😭