I often find myself sparking up conversations with girls or a girl at the bar.

Many times I'm having a great conversation and somewhere along the way the girl says she has a boyfriend, but we continue to talk. (I believe the BF part is genuine).

I will then either get invited to meet her friends or invite myself to meet them.

What kind of energy, vibe and openers should I use when meeting this new group.

On the one hand I feel its displaying high value that the friend is bringing me over but at the same time its difficult to bounce off the friend as I dont really know her beyond the 10 minute conversation we have just had.

Any thoughts?

Apply the same things you did to pick up the chick at the bar.
One more thing: always be positive