As many posters already said, I would not suggest you to convince her.
Furthermore, I have a simple rule. Luckily I have some experience in swinging and group sex, but I would never do it with my wife. Actually I would not do it with a girl I have a serious emotional attachement. I found group sex great fun. Oh may, I remember some fantastic group sex in german swinger clubs, but just a thought that GF I am emotionally firmly attached to me might replace me by someone from such orgy ... Honestly I couldn't stand it. Maybe it is a bit hipocritic of me, but that's how it is with my feelings. When sex is concerned I thing we must be honest in accepting our feelings.