you mean you have for N users N copies of the file, each encrypted with a different key, or one file that is encrypted with N keys and then you need N keys to decode?
[0]: http://security.stackexchange.com/questions/8245/gpg-file-si...
Assuming you've all the public keys in your keychain, then for each key you pass "-r <email>" for the encryption, like "gpg -e -r foo@bar.com -r baz@bar.com plain.txt", and for decryption you do something like "gpg --output plain.txt -d plain.txt.gpg".