The LINQ to SQL/Entity Models released with the SP1 of Visual Studios does not innately support Many-To-Many DB realtions. This can be shown when working with the Designer, and trying to draw a new association between to table and selecting the relationship type. Notice it only has one-to-one and one-to-many?
However, you can still simulate it with the following LINQ statements :
WebSiteMasterzDataContext webDb = new WebSiteMasterzDataContext();
var query = from s in webDb.Staffs
join pro in webDb.Staff_Projects on s.StaffId equals pro.StaffId
where pro.ProjectId == ProjectId
The code above effectively does a join between the Staff and Project table through the many-to-many table Staff_Projects.
Hope this helps,