You might start out by a BIG visual aid of a simple family tree, your own or fictional, and talking a bit about how much is lost - witness all the "celebs" on WDYTYA. You might ask them if they've ever watched that; if they know of any recorded tree related to them - get them talking, anecdotes etc.
Then explain that's exactly what they can best pass forward to family historians - their own or the wider world. Talk about the recently televised VE day memories recalled - so easily those could be lost.
Read out a few anecdotes or significant event memories and blu-tack them in the appropriate spaces on your Vis Aid Tree. Add photos - pointing out that pencilled in details on the rear can save a lot of time. Possibly have a participation event for them - a sheet with a few photos (females are easier) on it that they can try to pin down the dates of ( a little prize?) and discuss how they can elicit evidence from pics. You could also add photos of places - preferably in the locality, old pics, and also show the same places in the present day.
That load of stuff up on display should demonstrate well how additional information can flesh out the bare facts of born, married, bred, died, and enrich it.
Then you can go into how your audience can do this - show a pic of or have a box of muddled old photos, with no information. Show albums. Explain how captions in albums, and pencilled notes on the back of pictures can be really helpful - dates and places, too, as well as names. "Cousin Jean" isn't as helpful as "My cousin Jean Bloggs, aged 18, at Blackpool, summer 1972" - and possibly their own name or initials. Explain too how free sites like RootChat can be of great help to family historians (we all know not all "elderly" are at all phased by computers, don't we?) and suggest how they may like to start. You could even distribute a sheet with either a fill-in-your -own -family mini - tree, and/or a few questions like:
* Where were you born?
*When/where were you baptised?
* Where were your parents married?
*What were your grandparents' names?
* Where were your grandparents married? When?
*What do you remember about where you first lived?
*Describe what one of your grandparents looked like?
- I'm sure you'll think of far better things yourself.
To wind it up, you could ask them if they'd like to have another meeting say a month later, and each bring a small local memory, a story, photo or something, with them then.
-Then go for a good drink!