Dr. Salah Sultan, President of the American Center for Islamic Research, Columbus, Ohio, and Member of the European Council for Fatwa and Research, states: “The basic rule is that women are supposed to cover everything except the face and hands. However, when women are alone a woman is allowed to show her hair, neck up to the chest, ankles, and arms before other women. Therefore, it is allowed for women to wear clothes showing cleavage during women-only parties, provided that no picture could be taken by women who may show them to other people, and no videotape could be recorded, but only mahram if they wish to take pictures or close female friends who will not show these pictures to others. The basis for this is aya 31 in surat Ar-Rum where Allah says: “And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what (must ordinarily) appear thereof; that they should draw their veils over their bosoms and not display their beauty except to ….”
Having said this, there is another discussion in Islamic Fiqh regarding showing these parts before a non-Muslim lady, but I could say that the whole issue is related to honesty and trustworthiness. So, if the woman is trustworthy and she will respect the Islamic teachings of privacy and not showing the `awrah before men or describing women to other non-mahram people, then it is allowed, otherwise it is not allowed.”