If the woman is one of your Mahrams, there is nothing wrong with you leading her in prayer and it will be counted as prayer in congregation. But if she is a non-Mahram, and if your leading her in prayer means that you will be alone with her, then in this case your leading her in prayer is prohibited.

1- When a person prays behind an imam, four scenarios may apply: 1- doing a movement before the imam, 2- delaying in following the imam, 3- doing an action simultaneously with the imam, and 4- doing a movement immediately after the imam. 2- Following the imam is the Sunnah and what it means is that a person starts to do the actions of prayer immediately after the imam starts them, but without doing them at the same time.