Technoguy - seems to me like you're not comfortable about the idea, so the answer is DO NOT DO IT.

Don't allow yourself to get painted into a corner, where she's effectively emotionally blackmailing you into doing something you don't want to do.

You are both young, and you have plenty of time to experience these things later in life, if you wish, when you are both at a place where you feel you can deal with this emotionally.

Also, I would warn against anything that will put a strain on your relationship at a point where it is obviously a little shakey (you mention you have just got back together). I would also say that unless a couple are both totally and utterly into the idea and without any doubts or insecurities, it WILL cause more problems in the relationship, than already exist.

Finally, as for the experience making her a better writer, there are some excellent erotic writers here on this site, and a lot of stories about group sex experience. Most of them have probably been written based purely on fantasy, not on experience, so I really can't see that she will be impeded by in her writing by her lack of experience.

Hope this helps - sorry it's an essay as usual!

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