Hi Alice,
Regarding your matter, you could practise the collection of payment upon arrival at the shoot. I encountered 1 Aussie model who practised this when I hired her. Honestly, I was a little put off by her but I could also understand that she probably had a bad experience with others who hired her but didn't pay.
Since asking for full payment upfront can be unpleasant, ask for at least half (in cash) before you begin. For the kind of work you are doing, I would have thought you do this.
Why do I get the impression that many group photo shoots organised by ClubSnap members are on a no-profit basis? It's logical enough that models are paid but if the organiser earns absolutely zero (or worse, miscalculates and ends up spending his own $$ and makes a loss), why bother organising shoots then? What's really motivating such an activity? To win a popularity contest among "the brotherhood of men"? Duh!