Recursion On Returning Table

I have a multi-level folders table, named folders with attributes of folder_id, parent_id and user_id, and have another table, users, contains all the user_id. I need to list all the users for each folder which has parent_id = 0 and its all sub-folders' users into one table. I have created a function to return a table with folder_id and user_id for one single folder. However, I don't know how to use this function to get the sub-folder's users and merge them together as one single table.

Here is my function:

CREATE FUNCTION [dbo].[FolderUsers] (@fid int) 
RETURNS Table  AS 
Return
 (select folder_id, f.user_id
 from folders f, users u
 where f.user_id = u.user_id and folder_id = @fid)
Go

where @fid is the top folder with parent_id = 0 at here, the next level sub-folder's parent_id would be = @fid.

I am thinking to have recursive call from the parent_id = @fid that returns another table and have to concatenate to the called table. I have been thinking of store procedure, "insert into" and so on, but don't know how to implement it.

Do you have any good inspiration for me? Thank you in advance!