That sounds like a lovely party, what a good idea.
If I were you I'd make some examples of what they could make before they arrive, have them on show otherwise they'll rush in, shove any old thing together, create some hideous interesting pieces then be bored.
I think I'd maybe limit them to three or four things each, a necklace and two bracelets sounds about right.
Elastic rather then thread too, pre-cut into suitable lengths.
Ebay is brilliant for beads, very good value and all sent to you so very convenient.
My DD and I made sun catchers for hanging in front of her window out of glass beads bought from ebay, very simple but pretty, you could make them too.
I think a jewellery party would go well with a trad tea party; cups and saucers, a teapot, neat sandwiches, delicate cakes on a stand etc with all the girls wearing their new jewellery.
If I think of anymore good ideas I'll be back.