SELECT Receivers FROM myTable
UNION
SELECT Payers FROM myTable

--Setup Sample Data
declare @transactions as table (Receivers char(1), Payers char(1))
insert into @transactions(Receivers, Payers)
values('A' , 'A')
,('B' , 'A')
,('A' , 'B')
,('A' , 'D')
,('B' , 'D')
,('C' , 'A')
,('B' , 'E')
,('A' , 'E')

--Actual work statement
select distinct [Group]
from (
		select Receivers [Group]
		from @transactions
		union all
		select Payers [Group]
		from @transactions
		) c

declare @transactions as table (Receivers char(1), Payers char(1))
insert into @transactions(Receivers, Payers)
values('A' , 'A')
,('B' , 'A')
,('A' , 'B')
,('A' , 'D')
,('B' , 'D')
,('C' , 'A')
,('B' , 'E')
,('A' , 'E')

SELECT RecPyr
FROM @transactions
CROSS APPLY (VALUES (Receivers),(Payers)) a(RecPyr)
GROUP BY RecPyr