This is a very poor database design, if I understand the question correctly. Here is one way:
select *
from UserTable ut
where exists (select 1
from RosterTable rt
where rt.UserA = 'Y' and ut.username = 'UserA' or
rt.UserB = 'Y' and ut.username = 'UserB' or
rt.UserC = 'Y' and ut.username = 'UserC' or
rt.UserD = 'Y' and ut.username = 'UserD'
);