using .NET 3.5

I am trying to use IQueryable here and dont want to have to convert to a list/collection in order to do something of the following:

I am querying a table. it has UserID.
I want to be able to do a left outer join from the Users table so I can obtain Username.

how can I do this?

currently, the objects I have expose UserId, which is fine, and then I want to be able to obtain Username at some stage and populate it into an IQueryable entity which I have (which 99% has the same properties as my other objects with this one exception which is what I need)