Hadrat Um Atiyara narrated that she heard the Holy Prophetsas say that women:
[Women] should come out and take part in the good deeds and in the religious gatherings of the believers. But the menstruating women should keep away from the Musalla [place of prayers].
(Sahih al-Bukhari, Book 25, Hadith 133)
Hadrat Maimunahra narrates:
The Messenger of Allahsas used to lay his head in the lap of one of us and recite Quran while she was menstruating, and one of us would take the mat to the masjid [mosque] and spread it out when she was menstruating.
(Sunan an-Nasa’i, vol. 1, Book 3, Hadith 385)
Hadrat ‘Aishahra narrates:
The Messenger of Allahsas ordered me that I should get him the mat from the mosque. I said: “I am menstruating.” Hesas said: “Do get me that, for menstruation is not in your hand.”
(Sahih Muslim, Book 3, Hadith 12)
During the menstrual cycle, women may visit the mosque if they need to give or take something from there, however they are not permitted to sit there. Had they been permitted, why would the Holy Prophetsas instruct women going for Eid to remain separated from the area where prayers are offered? Hence, women are not allowed to sit in the mosque during this phase.
If there is a Jama‘at which does not have a place apart from a mosque to accommodate Lajna members for various meetings, then in such cases women who are in their menstrual cycles may not attend these meetings. A Lajna member is not obligated to attend these meetings but it is obligatory upon her to obey the commandments of Allah. Else, alternative sitting facilities may be provided for such Lajna members at a place where prayers are not offered. The area where shoes are usually taken off before entering the mosque can be cleaned and arrangements may be made there for this purpose.