Is it permissible for women to pray in congregation behind an Imam who is unaware of their presence, and does their prayer become invalid because of this?

It is permissible for the congregant to follow the Imam if the Imam is outside the mosque, provided that the congregant can see the Imam or those behind him, or if the rows are connected, or if it is possible to follow by seeing or hearing the voice. The Imam's awareness of those praying behind him is not a condition.