Does it import my WhatsApp contacts automatically?
No, and we won't say it does. You add the guest names to the list yourself, and you send the link through WhatsApp as usual. The difference is that replies then gather on one list instead of across your chats.
Do the people in my chats need an account?
None. They open the link in WhatsApp's browser, read the invitation, and reply in a couple of taps. There is no sign-up before a yes reaches your list.
How do I stop the details getting lost in the thread?
They live on the page, not the message. The address, the time, and the plan stay in one place, and if anything changes you edit the page rather than re-posting into every chat.
Can guests say they are bringing someone?
Yes, within the plus-one limit you set on the invitation. They name the companion on the reply, so the extra place shows on your list with a name attached.
What about relatives who will only text me?
Record their answer on the same list yourself. Whether they tap the link or tell you over the phone, the count comes from one place, so nothing double-counts or slips.