You can´t return more then 1 row in a select subquery. To this case you have to use the CASE expression.
You have to do something like this:
SELECT (m.First_Name + " " + m.Last_Name) AS Member,
case
when f1.my_email is null
then f2.my_e_mail
else f1.friend_email
end as email,
m.First_Name AS Name
from members m
left outer join friends f1
on m.email = f1.my_e_mail
and f1.friend_e_mail = ?
left outer join friends f2
on m.email = f2.friend_e_mail
and f2.my_e_mail = ?;
Thys way it will return a row for each friend. You can see that the names of the fields are not so good, try to refactory it. my_e_mais isn´t so representative.