According to the Shaafi school of Sunni thought a woman can lead another group of woman in prayer. They have based their opinion on a Hadith which Imam Al-Shaafi has narrated in his Musnad with a Hasan chain that 'Aisha and Umm Salma (Allah is pleased with them) lead a group of woman in prayer and stood in the middle of them.
Imam Al-Nawawi has mentioned this in his famous book of comparative fiqh known as المجموع شرح المهذب. For those who known Arabic they can visit this link to read the discussion in detail in the book.
This opinion, of the permissibility of a woman leading other woman in prayer, was the opinion of the following scholars of Islam
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عائشة
أم سلمة
عطاء
الثور
، الأوزاعي
الشافعي
إسحاق
أبو ثور**