Create user for all existing databases

USE master;

DECLARE
    DBNames CURSOR
FOR
    SELECT
        NAME
    FROM sysdatabases

OPEN DBNames

DECLARE @Name varchar(50)

FETCH NEXT FROM DBNames
INTO @Name

WHILE (@@FETCH_STATUS <> -1)
BEGIN

    Declare @SQL varchar(255)
    Print 'Processing: ' + @Name
    BEGIN TRY
       Select @SQL = ' USE ' + @Name + ' CREATE USER [ldbamonitor] FOR LOGIN ["LOGINNAME HERE"] EXEC sp_addrolemember N''db_datareader'', N''ldbamonitor'''
       EXEC(@SQL)
    END TRY
    BEGIN CATCH
        select ERROR_MESSAGE() AS ErrorMessage;
    END CATCH

    FETCH NEXT FROM DBNames    INTO @Name
END

CLOSE DBNames
DEALLOCATE DBNames