Update a dynamic number of rows with an amount proportionally
You can solve it this way:

declare @a table 
(groupID int, reservationID  int, [Date] date, Amount decimal(10,2));

insert into @a values 
(26,      1,              '2018-06-01',  NULL),
(26,      2,              '2018-06-01',  NULL),
(26,      3,              '2018-06-01',  NULL),
(27,      4,              '2018-06-04',  NULL),
(27,      5,              '2018-06-04',  NULL);

declare @b table 
(groupID  int, [Date] date,  Amount decimal(10,2));

insert into @b values
(26,       '2018-06-01',      60),
(27,       '2018-06-04',     100);

with cte as
(
select a.*,
       b.Amount / count(a.reservationID) over (partition by a.Date, a.groupId) new_amount
from @a a join @b b 
        on a.Date = b.Date
        AND a.groupId = b.groupID
)

update cte
set Amount = new_amount;

select *
from @a;