Forum Moderators: open
If it's just to open and save the file on their local machine then a simple link to the .xls file would suffice.
If it's to open, modify and save back to the server then it gets more interesting! You could do this inline with the use of an ActiveX object, but the simplest thing may be to allow the users to download the file and make their changes and then upload the file again, overwriting the original or being created on the server as a new file.
If you allow users to overwrite the original file, then you will also need to lock the file when one person has it "checked out" - this will prevent overwriting new data.