Who is going, and who is driving
The part that is not a calendar: saying you will be there, sorting out who drives, and staying out of the way when nobody is going.
Saying whether you are going
Every event has three answers: I'll be there, Maybe, and Can't make it. Tap one and it saves — there is no separate save button, on your own screen or on an invite link.
Not answering is not the same as saying no. An unanswered person is simply unanswered, and ThereFor never counts silence as a decline.
Rides
A ride is split into the legs that actually go wrong: I'll drop off, I'll pick up, I'll drive, and I'll cover. Each one can be claimed by a different person, which is usually how a Saturday really works.
Anyone on an invite link can claim a leg. That is the point — the grandparent who is already driving past the rink does not need an account to say so.
When nobody is going
If every assigned person has said they cannot make it, the event stops asking. Who's going and the ride legs disappear, because there is nothing left to coordinate.
It is always reversible: the people actually assigned to the event keep their own controls, so changing your mind brings everything back.
Following the people you care about
Someone on an invite link can choose who they follow, so a grandparent with six grandchildren sees the two whose games they actually attend rather than everything at once.