All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The minimum number of people for a congregational prayer to be valid is two.
Al-Bukhari wrote a chapter entitled "Two or more people form a congregation". The evidence about the above is what Al-Bukhari and Muslim reported on the authority of Maalik Ibn Al-Huwayrith who said: 'I was about to leave the Prophet and he told me and another man with me: "You and your friend have to make Athaan (call for the prayer) and Iqaamah (establishment of the prayer), and the one who is older should lead the prayer."
So praying in congregation has to be established whether there are two men, or two women, or a man and a woman, or a man and a boy at the age of distinction, (i.e. seven and older). The difficulty is if those women are alien to that man. Anyway, the prayer is correct and this situation is not considered a forbidden seclusion.
However, it is forbidden for a man to be in seclusion with a woman, or several men to be in the presence of one woman. It is also forbidden to be with a woman who is in the company of a young boy before whom one is not usually ashamed to sin, like a boy of two or three years of age.
There is no harm, however, for a man to be in the presence of several women, as it is mentioned in the question, especially with the presence of two 12 year-old boys before whom one would usually be ashamed.
An-Nawawi said: 'The correct opinion is that it is permissible for a man to be in the presence of several women.'
Allaah knows best.