How to get a group where the count is zero?
The reason your query did not work as intended:

Inner join gives you the intersection of 2 tables. In your case, there was no entry for 5th street in your users table and that is why join did not produce any entry for that. 

Outer join (right or left) will give the result of inner join and in addition all non-qualifying records from the left or right table depending on the type (left or right) of  outer join.

In this case, I put Street on the left of the join and used left outer join as you wanted all streets (even count is zero) in your result set.

Change your select query to this.

SELECT S.Name AS Street,
       Count(U.Username) AS COUNT
FROM Streets S
LEFT OUTER JOIN Users U ON U.Streetid = S.Id
GROUP BY S.Name


Result