Is it permissible to ululate (zaghareed) at a marriage contract ceremony designated for women only, and is that considered a sin? And how can a women's gathering be held within the bounds of Sharia?

If "al-halahil" refers to ululations, then their ruling has already been stated. As for how to hold a special celebration for women, it is permissible for them to sing and beat the duff (tambourine), and do everything that expresses joy and happiness, within the bounds of Islamic law, and without committing forbidden acts.