Either one of you can do it, @Liz-And-Scott0 . I usually ask the guest to do it, so they learn how for next time, but sometimes they "can't find the button" and so then I do it for them.
If you can't find the button, it is at the top of that guest's message thread in your inbox. Open the thread and then look for the "Change or Cancel" button. Just follow the prompts to change the number of guests.
The guest will have to accept your changes in order for them to take effect.