I'm gonna donate, but ... is it such a good idea to add so many people? Because in the end there will be not a lot of money going around to each and everyone of them. If we had more users donating it would be a different matter.

Wouldn't it be better to have a rotation? Let the association members decide which five persons (for example) should split the money this time and then vote for 5 others the next time. This way the five recipients will get quite a lot of money, which hopefully with motivate then more. I would believe everyone think that's more rewarding than everyone get to share some 4 euros.