PGP uses the pubkey to encrypt a randomly generated symmetric key, because it's much faster to asymmetrically decrypt a small key and then symmetrically decrypt the entire message. For multiple recipients, this symmetric key is simply encrypted once for each recipient pubkey.
Basically - generate a session key, encrypt the session key with for each recipient, at the end add the text of message encrypted with the session key.
AFAIR GPG will generate a small bit of random data and use that to encrypt the message. For each recipient, tt then encrypts that new key (which is only ~100 bytes) with that recipients private key.
Minor but significant nit: that each of the recipients can open.
The reason it's significant: there are encryption protocols which require some subset m < n, but generally m > 1, such that a quorum of members must assemble or cooperate to read a message. If m = n, then you indeed have a situation where all the recipients must cooperate (that is: all are present or contribute their keys) to read a message.
