I have a preference, maybe even an exclusive one for a FFM menage a trois.
Dunno about another guy being involved...two girls would be more exciting for me
Anyway, I hear threesomes ruin a lot more relationships than they maintain or improve. Also, itd be bad with a guy from work, who she would be seeing often etc...I can imagine him asking for a private 'rehearsal' with your gf.
Best done with a stranger I think, or two girls! But its up to you. If you're nervous though, don't do it.
Nerves + sex don't work well together. What if you can't get hard and its just the guy from work getting his rocks off with your gf? She might like it a bit too much etc.
As for what to do, just do what you feel comfortable with.