This would most likely mean that you're are "the other woman", or "loose". Neither are titles to strive for.
Move your hips having your groin area on her.
I assume you are referring to inline skating. Yes you can grind without a grind plate. The plate is used to slow down the wear of your skate frame.
you basically just get behind the girl and hopefully someone follows.
if your girlfriend has a girlfriend, then she might not fully understand the concept of a faithful relationship, and is clearly Bi-Sexual. just let her know easily how you feel about that, and ask...