In the Name of Allah. Peace and blessing be upon our Prophet Muhammad and upon his family and his companions, all of them. Peace be upon you, along with the mercy and blessings of Allah. I live in a western country in a community with a good number of Muslims. Praise is due to Allah! We have a mosque where there is a place for women. But sometimes a woman or two come and pray with us in the place dedicated for men. Is this permissible? What is the ruling of this case, considering that there is no curtain between us and them? May Allah reward you! Peace be upon you as well as the mercy and blessings of Allah.

All praise is due to Allah alone. Peace and blessings be upon the Messenger of Allah. And upon you be peace, together with the mercy and blessings of Allah. The basic principle is that a woman has to pray in the lines of the women. Having a curtain is not a condition, but it is more proper to have a curtain or a screen. In the time of the Messenger of Allah (peace and blessings be upon him) women prayed behind men, as mentioned in the authentic ahadith, without screens or curtains. Sometime men’s clothes were not long enough and women were ordered not to raise their heads from prostration until men raised their heads first. There are undeniable hadiths to this effect. See Musnad Ahmad (22911), Saheeh al-Bukhary (362), Saheeh Muslim (441.)
Moreover, in the noble Two Holy Mosques women pray in the lines behind men. But if a woman prays in the same line with men, this is not permissible. The point concerning the permissibility of women to pray behind men without a curtain is based on the purity of souls and the avoidance of dubiousness. Thus, women have to pray in the places dedicated for them, whether behind a curtain or not. If a woman prays isolated from men’s lines, her Prayer will be valid. But she should not mingle with men or be allowed to join their lines. Allah knows best.