We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may Allaah preserve him, who answered as follows:
It is better for them to pray in jamaa’ah, if possible, but the reward of twenty-seven times more applies only to men, because praying in jamaa’ah is obligatory for them.
“The correct view is that for women to pray in jamaa’ah, and for a woman to lead other women in prayer, are things that are permitted in Islam and indeed are mustahabb for them. The evidence (daleel) for that is the hadeeth of Um Waraqah that we have referred to above, in which it states that the Prophet (peace and blessings of Allaah be upon him) gave her permission to lead the (female) members of her household in prayer… It is also confirmed by the fact that some of the Mothers of the Believers led other women in prayer. It was reported from Laytah al-Hanafiyyah (from the tribe of Banu Haneefah) that ‘Aa’ishah the Mother of the Believers led them in the obligatory prayer. And it was narrated from Tameemah bint Saleemah that ‘Aa’ishah the Mother of the Believers (may Allaah be pleased with her) led women in Maghrib prayer, and that she stood in the middle of them and recited Qur’aan aloud. It was also narrated that Um Salamah the Mother of the Believers (may Allaah be pleased with her) used to lead women in prayer in Ramadaan, and that she stood with them in the row. And it was narrated that Ibn ‘Umar used to tell one of his slave women to lead his womenfolk in prayer in Ramadaan.”