My mother does not use WhatsApp at all. What do I do?
Then the channel is whatever she actually uses — a phone call, an email her grandchild checks with her, a card. The invitation page still earns its place behind that: call with the news, send the link for the detail, mark her answer on the list when she gives it. Any road is fine as long as the yes lands on the list.
Two grandparents share one phone — can they both reply?
One reply for the household is the cleanest. The button takes a name; your limits decide whether a plus-one is included. If the household answers as a bloc, count it as one answer for both names, which is what the list is for.
Is it safe for older guests to type their details into a link?
Yes, in the ordinary sense — the reply is a short form, and the page asks only what the event needs: a name, whether they are coming, a meal choice if you ask for one. Nothing on a guest's phone is read by answering, and a forwarded page shows only the invitation, never the guest list.
Should I avoid group chats with older relatives entirely?
Not entirely — but no significant answer should be expected to survive one. Use the group for warmth and noise; use individual messages for the invitation itself, with a gentle ask to reply on the page rather than in the chat.
What if she answers by phone instead?
Count it. The live list exists so a yes taken by mouth can sit beside a yes taken by thumb. Mark her down, and let the deadline reminder reach only the names still genuinely blank — one of the gentler uses of a reminder is confirming that the aunt who said yes at lunch is safely on paper.