Can I invite more people than the venue seats?
Set your list and put the plus-one rule on the page rather than in a separate conversation with each guest. If the room is tight, say what you mean: seats for thirty, so tell us who is coming and we will confirm. Then the list is the number, not the optimism.
Should the theme be a surprise to the birthday person?
It often is. The honest tool for that is access: send the link to everyone but the guest of honour, and password-protect the page so a stray search does not hand it over. A draft is invisible to guests, so you can keep editing right up to the day.
Do guests have to answer for their partner separately?
No. One reply can carry a plus-one within the limit you set, with names attached, which is the difference between fourteen guests and twenty-eight. If the venue needs a hard number, that limit is how you keep it.
How do I end up with a playlist I did not build alone?
Add the playlist link to the page and open the song request section before the day. You see what the room wants early enough to cut what it does not, and nobody has to negotiate with you at the speaker.
Is there a limit to how many people can reply?
Nothing to plan around. You start free and upgrade as your guest list grows, so a list of twenty-five and a list of ninety are both workable. The plans are set out on the pricing page.