Now that's some tea party ... 600 people!! WOW!!!!!
Could you get a bunch of people together and make them together? Probably wouldn't take long if you had some cutting, some sewing, some pressing .........
Could you get a specialty tea shop to donate the tea?
At times I've seen little organza bags with ribbon draw strings at the $ $tore that'd be perfect for your purpose.
From the other perspective .... do they "need" a take home gift?