SELECT DISTINCT Person.ID, ...
FROM Person LEFT JOIN Manager
ON (Manager.ID = Person.ID OR Manager.ID = -1))
WHERE (...)

id  name

1   Person One
2   Person Two
3   Person Three
4   Person Four
5   Person Five

id  name
-1  Admin
2   Manager One
3   Manager Two

SELECT DISTINCT *
FROM Person LEFT JOIN Manager
ON (Manager.id = Person.id OR Manager.id = -1)

Person One  -1  Admin
Person Two  -1  Admin
Person Two  2   Manager One
Person Three    -1  Admin
Person Three    3   Manager Two
Person Four -1  Admin
Person Five -1  Admin

insert into #test (test1id, test)
select 1, 'Judy'
union all
select 2, 'Sam'
union all
select 3, 'Nathan'

insert into #test2 (test2id, test1id, test2)
select 1,1,'hello'
union all
select 2,1,'goodbye'
union all
select 3,2,'hello'

select * from #test t
left join #test2 t2 on t.test1id = t2.test1id
where test2 = 'goodbye'
--result set
--test1id   test    test2id test1id test2
--1 Judy    2   1   goodbye

select * from #test t
left join #test2 t2 on t.test1id = t2.test1id
and test2 = 'goodbye'
--result set
--test1id   test    test2id test1id test2
--1 Judy    2   1   goodbye
--2 Sam NULL    NULL    NULL
--3 Nathan  NULL    NULL    NULL

select * from #test t
left join #test2 t2 on t.test1id = t2.test1id
where test2id is null
--result set
--test1id   test    test2id test1id test2
--3 Nathan  NULL    NULL    NULL