1. A faasiq (one who knows the deen but does major sins openly).
2. A bidati (an innovator in religion).
3. A blind person except if he is the best one in the group.
4. An ignorant person of the laws of namaaz when there is an Alim present.
5. A slave
6. The one who was born out of wedlock.
It is also makrooh to have a jamaat of only women and no men.
If there is a women only jamaat the one who will be imam will stand in the middle of the saff and not forward.
Because of the fitnah of the times it is also makrooh for women to come to the masjid and pray behind a male imam.