A girl who someone dates to get closer to her friend.
With the ultimate goal of dating her friend, you date girl A. When that relationship inevitably fails, you can move onto girl B, whom you have been getting to know throughout the span of the first relationship. Girl A was the step-girlfriend used to get to girl B.
A man who is married to a woman who has a daughter with another man in which he then proceeds to date the daughter of his wife; the step-daughter is now the step-girlfriend.
I told my step-girlfriend to wipe the cock tears from her face before her mother got home.
A woman who dates a man that has children by another woman. The woman is the girlfriend to the father of the children and thus a Step Girlfriend to the children.
The children left their Mom's house to go visit their Dad and Step Girlfriend for the weekend.