PGP uses hybrid encryption. That means that the public/private key system is not used to encrypt the whole message. It is only used to encrypt a symmetric key which is used to encrypt the actual message.
When you address a PGP message to multiple recipients, you only encrypt the message once with a newly created symmetric key, and then encrypt that symmetric key multiple times, once for each intended recipient. That way each recipient can decrypt their personal copy of the symmetric key and then use it to decrypt the message.
Your message would grossly oversimplified look like this:
Symmetric key encrypted with public key of user A
Symmetric key encrypted with public key of user B
Symmetric key encrypted with public key of user C
Symmetric key encrypted with public key of user D
Symmetric key encrypted with public key of user E
Message encrypted with symmetric key
By the way: Hybrid encryption is also used when you send a message to only one recipient. The reasons is that using a different symmetric key for each transmission prevents an eavesdropper from noticing when you write the same message to the same recipient twice. It is also better for performance, because most symmetric encryption algorithms are much faster than most asymmetric ones.
