Если бы я правильно понял, это бы сработало:
with s as (
select
SUM(rate) rate,
SUM(nights) nights,
SUM(rate) * SUM(nights) subtotal,
(SUM(rate) * SUM(nights)) * MIN(tax) tax,
SUM(adults) adults,
SUM(fee) fee,
((SUM(rate) * SUM(nights)) * MIN(tax)) + SUM(fee) total,
from table
where group = @group_code
group by rate
), sorted as (
select s.rate, s.nights, s.subtotal, s.tax, s.adults, s.fee, s.total, ROW_NUMBER() over(order by s.rate) lp
from s
)
select sorted.rate,
sorted.nights,
sorted.subtotal,
sorted.tax,
sorted.adults + IIF(lp = 1, -2, 0),
sorted.fee,
sorted.total
from sorted