Thank you very much for the informative responses.
NovemberRain: I understand that women express closeness and love to their girlfriends more than men. It could certainly just be that I'm thinking wishfully and seeing things that aren't there, or at least I would not doubt that as a possibility. I still have a slight feeling that there could be some attraction growing, however. Another example is that they occasionally cuddle on the couch or nap together in my bed, and at one point when our friend was on her hands and knees, petting our cat, my girlfriend gave our friend's rear a smack. Our friend seemed a bit flustered and my girlfriend jokingly said "If you're going to stick it out there, it's gonna get smacked!" This was somewhat shocking to me, as it is definitely not like my girlfriend to do something like that. Once again, maybe none of that is intimate at all, in any nature, and I'm trying to see it as such. I guess I don't know!
idealist: I absolutely understand the importance of doing more research, processing my own feelings and coming to an understanding of what I truly want. I think I need to do more of that before I move forward. Also, I feel that I share your natural gift of being able to influence people, mostly because I am a very logical person and I have a way of helping people get through their fear and see things rationally, the way I do.
ThatGirlInGray: I think you're absolutely right about discussing polyamory with her first, before discussing this situation with our friend, although she actually did bring it up at one point a few months back. We were having some issues, directly related to my feelings for our friend, and at the time, before I really had a grasp of what polyamory was, I mentioned to her that I was struggling with feelings for others, like I wasn't sure if our relationship could last forever with just she and I. She processed what I had said and told me that we could try threesomes but that she doesn't think she can handle polyamory or open relationships due to her jealous nature. I will say, however, that her issues with jealousy (along with all of her anxiety related issues) have come such a long way in the last eight years. I'm not sure if the idea of polyamory actually bothers her as a whole (or if she's even taken the time to sit and think about it) but I do have a lot of confidence that she can get past jealousy, just as many people who are easing into polyamorous relationships learn to get past theirs.
As far as me helping her get to a point where she can want or accept this, I very much apologize for expressing it in such a way that seemed like I intend to force this on her. What I really want is just to figure out if this is something she could actually consider or not. If I see a sign that she could or would consider something like this, or at the very least if she seems even somewhat interested in it, that's when I would move forward and look into more resources that can help her learn.
I think right now, my major problem is that I have no idea how to bring this up naturally, in a way that won't scare her or make her question why I'm asking about it. I want to introduce her to this and feel her out, but I just don't know where to start.