Most parties are low stakes about who shows up. A farewell is not. Someone is leaving, maybe for a long time, and the night is a chance for the people who matter to them to be in one room before they go. Getting turnout right is the whole job, because you usually do not get a second attempt.
Frame it as the last chance
The single most effective thing you can do is be honest about what the event is. "Sarah leaves for Lisbon on the 28th, this is the last time the whole crew can send her off properly" lands differently from "drinks on Saturday." People deprioritise generic plans. They show up for moments they cannot get back. Tell them clearly that this is one of those.
Invite wider than the usual crew
Farewells are the time to reach beyond your normal group. The person leaving has people across different parts of their life, and many of them will want to be there even if they are not in your usual chat. This is exactly where the no account invite earns its place: you can invite the old school friend, the former colleague, the cousin, by email or a simple link, and they can say yes without signing up for anything. Do not let the guest list shrink to only the people who happen to use the same app.
Give people a way to mark it
A send off benefits from one small ritual: a toast, a card everyone signs, a group photo, a moment where someone says the thing out loud. It does not have to be sentimental or staged. But a farewell with no marker can feel like an ordinary night that happened to be the last one. A single deliberate moment gives people the chance to actually say goodbye.
Get the headcount right for the toast
Because turnout matters more than usual, lean on reminders. People who would happily come can still forget the date when life is busy. A nudge a few days out and a reminder the day before, with the time and place, catches the people who meant to come and lost track. For a farewell, the gap between meant to come and came is the saddest one to leave on the table.
Let the person be present
The host's job at a send off is to handle everything so the person leaving does not have to. They should not be the one chasing RSVPs or worrying about whether enough people will turn up. Set it up, manage the list, send the reminders, and on the night, let them just be there with their people. That is the gift, more than any party detail.