I think this is what you're looking for...

Providing easily consumable sample data makes it easier for people to help and more likely...

--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