If we have 4 records, we want to concatenate all 4 records and the last separator should be "&" symbol. We can achieve this by using row_number() function and then decide whether the separator is "," or "&".
For example:
declare @s nvarchar(max)
create table #tb1 (
email nvarchar(255)
)
insert into #tb1 values ('a@a.com');
insert into #tb1 values ('b@a.com');
insert into #tb1 values ('c@a.com');
insert into #tb1 values ('d@a.com');
select
@s = coalesce(@s
+ case when rowidx < cnt then ',' else ' & ' end
+ email, email)
from (
select
rowidx=row_number() over( order by email )
, email
, cnt = (select count(*) from tb1)
from #tb1
) as a
select @s
drop table #tb1
