If a woman attends prayer in congregation in the mosque, and the people in the mosque decided to put prayers together because of rain, it is permissible for her to put the prayers together, following the congregation, even though in principle she is not required to pray in congregation. She may also pray Jumu`ah prayer with two rak`ahs, following the congregation, even though she is not obliged to attend Jumu`ah prayer, because something may be valid when connected to something else that is not valid when it is on its own. This view was stated clearly by some of the Malikis.
Abu Bakr as-Siqilli (may Allah have mercy on him) said: There is a difference of opinion regarding a woman whose house is next to the mosque, so she always prays with the people. It was narrated from Abu `Imran that she should not put prayers together, because she does not have the difficulty of going out to reach the mosque... someone else said: This woman may put prayers together with them, like one who is observing i`tikaf; the prayers are put together so that the people can attain the virtue of praying in congregation, and the same applies to the woman.
End quote from Al-Jami` li Masa’il al-Mdawwanah, 2/705.
Shaykh `Ullaysh (may Allah have mercy on him) said: It is permissible to put prayers together – in the event of rain – for one who is observing i`tikaaf or a stranger who is staying in the mosque, following the congregation.
End quote from Minah al-Jalil Sharh Mukhtasar Khalil, 1/422.
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said: Women should not put prayers together as men do, unless a woman prays with the men, and they put the prayers together. In that case, she may put prayers together following them. But in their houses, women should not put prayers together, because it makes no difference to the woman if she prays Maghrib at its right time and `Isha’ at its right time, or she puts them together, because so long as she is in her house, she does not experience any hardship or difficulty due to going out.
End quote from Al-Liqa’ ash-Shahri, 5/10.
Based on that, if a woman attends prayer in congregation in the mosque and they put prayers together because of rain, she may put the prayers together with them.
And Allah knows best.