I'm pretty sure basically everyone who plays the game agrees. Hoping that the Wedding pack addresses being able to invite friends and family of both sims who are being wed instead of just the one who sets it up, which would be an opportune time to also do the same for other types of parties that could have more than one sim hosting.
That said, another solution is to have the Sim who does know them invite them over before you set up the event and have the other Sim greet each of them.