You need to modify the recordset on the results page to use a JOIN Query to get results from the main table and the user name from the users table.
modify the select .. from statement of the recordset to a joined query, but do not change the where clause.
I do not know your table and column names, so cannot give the exact SQL, but the syntax will be:
SELECT mainTable.*, userTable.*
INNER JOIN userTable
ON mainTable.ForeignKeyToUserTable = userTable.PrimaryKey
Then open the bindings tab and esxpand the recordset, the username column from the users table should be listed, replace the userID in the results with the username.