If your girlfriend agreed to a threesome how do you convince her friend to join?

If you ever find the answer to this one PLEASE let me know.

We did have a threesome last year but it was MMF,she promised me she would return the favour,I'm still waiting.

I think it's not 'how you get her to',it's simply a matter of planning and being in the right place at the right time sometimes.

And lastly,just ask her!!