Quick answer
A spreadsheet works while a guest list is a list of names. It breaks the moment a name becomes a household with a plus-one, an allergy, a partial invitation and a reply that changes twice — because those are relationships, and a grid cannot hold them.
Where the grid starts to fail
The first fifty names are genuinely fine in a spreadsheet. The trouble starts when one row has to mean several things at once: a couple who share an address but eat differently, a guest invited to the evening but not the dinner, a plus-one whose name arrives three weeks after the invitation.
Problem 1: plus-ones are people, not a column
The moment you write "Sarah +1" you have created a guest with no name, no meal choice and no seat, who nonetheless has to be counted, fed and seated. Every downstream number — catering, seating, place cards — then needs a manual adjustment that lives only in your head.
Treating a plus-one as a real guest record fixes all of that at once: they get a name when the guest supplies it, a meal choice of their own, and they appear in the headcount without arithmetic.
Problem 2: households do not fit in rows
You do not want to email the Patel family four separate invitations, and you do not want to count their one reply as four confirmations. A spreadsheet can express "one row per person" or "one row per household" — not both, which is what you actually need.
Problem 3: replies arrive while you are editing
RSVPs come by email, by text, by phone and in person at the shops. Transcribing them into a shared sheet means two people editing the same cell, a stale copy on someone's laptop, and no way to tell whether the "yes" in row 84 is from last week or last month.
Problem 4: nobody can answer "who has not replied?"
It is a filter, in theory. In practice it is a filter over a column somebody filled inconsistently — "Y", "yes", "Yes!", blank, "maybe??" — which is why the chase always ends up being done by memory instead.
Problem 5: the sheet holds personal data with no plan for it
A guest list is a file of names, email addresses, phone numbers and sometimes health-adjacent notes about allergies. Emailed around as an attachment, it ends up in half a dozen inboxes with no way to correct or delete it. That is a real obligation being handled casually.
What to do instead
Keep the spreadsheet as the place you brainstorm names. Move it into a guest list tool the moment invitations become real, and let the structure do the work: households grouped, plus-ones as real guests, replies attached to the person who made them, and one filtered view of who is still pending.
Inviterio imports the spreadsheet you already have, flags duplicates instead of creating them, and keeps the list live from that point — including for events where you are collaborating with a partner or a planner who needs their own access rather than a copy of the file.