tldr: you’re going to need a bigger boat. And by bigger boat, I mean either another form platform or a different workflow.
The big project constraint I see here is that you would prefer that your coordinators use a form and not have to have Airtable accounts. This means two things. One, that you can’t use interfaces, because those require Airtable accounts, and two, that you’re going to need to look elsewhere for your form entry, because native Airtable forms aren’t going to let you do what you want to do. Even if you say, “Ugh fine I will make them get Airtable accounts,” they’re going to need to have paid seats in order to do what you want them to do in the interface. So I’m not sure that’s a great solution for you.
In general, Airtable forms allow a non-Airtable user to add *one* record in *one* table in *one* base. Now, you can use automations and syncs to then take data out of that record and copy it to other places, but what you need is different. You need to be able to create (at least) two records: one record in the Attendance table for the form submission and one in the Beneficiaries table for the Beneficiary that doesn’t exist yet.
You could play around with a form that has the activity and the beneficiaries as linked fields and then asking the user “How many new beneficiaries are there?” and having them enter some amount of data that would allow you to create an automation that creates records in your Beneficiaries table based on the data in the form submit. But that’s not something that the user will be able to use effectively at check-in. Or you would have a form that captures just the existing beneficiaries and ask new people to fill out another form (either on paper or on Airtable) that you can use to get their information and then create those records afterwards.
