Is it possible to encrypt a file once that can be opened by multiple users? I am trying to avoid having to create individual copies of a file and encrypt them multiple times, each time using a different destination user’s key. Thanks in advance.
I think it’s possible (not sure).
E.g. using Windows and GPGRelay:
The mailprogram sends the mail to GPGRelay and (I think, but never tried) encrypts the mail for every reciever.
If you try it, please report the result 
Well, I THINK that (at least when we talk about RSA)
it’s not safe to send the exact message to people with the same public key. Then it’s easy for a third party to find the key. And when we talk about “Encrypting a message once” we talk about using the same key.
But Tobias Paul’s solution is what you really need, fast and 100% safe and my post is just a stricktly theoretical comment, just found the question interesting. Sorry if I confused anyone…