I would definitely give the choice to your girlfriend. After all, the threesome can't happen without her consent. If she is hesitant or feels she doesn't know the friend well enough, you can take it easy and give them some opportunities to get acquainted; if she still isn't feeling the attraction, then you can discuss together what another option might be that would make you both feel comfortable and fulfilled. Communication is definitely key in any relationship, especially if you are bringing someone new into your dynamic! less