I've had several threesomes in the course of my life (both MMF and FFM), and in my opinion, the quality of the experience depends heavily on the individual people and attitudes involved. They can be a whole lot of fun, so long as everyone involved is capable of communicating their physical and emotional needs, approaches the situation with realistic expectations, and goes into the experience with a good sense of humour (because awkward, funny shit WILL happen, and it's best when everyone can just laugh about it and make it part of the fun).

I'd encourage you to sit down with your girlfriend and her friend and talk about it as a group. You may even find it beneficial to talk to your girlfriend's friend on your own (with her knowledge and permission, of course), especially if it's someone you don't already know well. Those conversations should help to give you a better idea if this is someone you'd even want to share an intimate experience with. You should be comfortable with him, and feel like there is good three-way communication between all of you as a group, because communication is extremely important for good threesomes (actually, it's important for good sex in general, but I feel like its importance kind of increases exponentially with every additional person you bring into the scenario). If your girlfriend's friend is not someone you've spent a lot of time with, it might even be helpful for the three of you to do some non-sexual group activities together, just to get a feel for how you all react off one another as a group, and to get a sense of your comfort level with just the chemistry of the group. For reference, most of my threesomes/group sex experiences (and all the good ones) have been with people I'd consider close friends. I'm not sure they would have been as enjoyable if we hadn't already known each other and been comfortable around each other.

I think it's also important for everyone to manage their expectations, especially if none of you have ever been in a group-sex situation before. It is NOT going to look anything like a threesome in a porno. Just trust me on this. I don't care how much of a demon in the sack any of you are, it will not work out like a porno, and expecting it to is just going to bring down the whole experience for everyone.

Sorry, I just realised that I've kind of dumped a lot of text on you. In a nutshell, my best advice is to get a feel for your potential threesome group in non-sexual situations, and to talk and communicate openly and honestly with each other. I feel like most other issues and concerns can be addressed pretty effectively so long as you lay down that groundwork. And if you ultimately decide that it's not something you're comfortable with, be sure to speak up and say so. Threesomes should be fun, so if you don't feel like it'll be fun for you, there's no reason to do it.

However it ends up playing out, I wish you the best of luck!