What app means here, plainly
When hosts search for an invitation app they usually mean one thing: stop juggling. Right now the invitation is a picture in a chat, the guest list is a spreadsheet, the reminders are a pile of apology texts, and the headcount is a memory. An app worth using collapses all of that into a single place you open when you need an answer.
One honest note before anything else: this is not a native mobile app you download from a store. It is a platform that runs in your phone's browser, so you can open the list on a bus, edit the page at a desk, or check who has arrived at the door, and it is the same list everywhere. Nothing to install, nothing to update.