with tbl as
(
	select [Week], Client
	from	(values (1, 'a'),
			(1, 'b'),
			(1, 'c'),
			(2, 'f'),
			(2, 'a'),
			(3, 'q'),
			(3, 'y'),
			(3, 'c')) as tbl ([Week], Client)
), tbl_gr as
(
	select	[Week], count(distinct Client) as new_Clients
	from	tbl
	where	not exists (select * from tbl t2 where tbl.Client = t2.Client and tbl.[Week] > t2.[Week])
	group by [Week]
)
select t1.[Week], sum(t2.new_Clients)
from	tbl_gr t1
		inner join tbl_gr t2
				on t1.[Week] >= t2.[Week]
group by t1.[Week]

with tbl as
(
	select [Week], Client
	from	(values (1, 'a'),
			(1, 'b'),
			(1, 'c'),
			(2, 'f'),
			(2, 'a'),
			(3, 'q'),
			(3, 'y'),
			(3, 'c')) as tbl ([Week], Client)
)
select tbl.[Week], count(distinct t1.Client)
from tbl 
	inner join tbl t1
			on tbl.[Week] >= t1.[Week]
group by tbl.[Week]