Hey man,
A few years ago I offered my girlfriend a similar experience.
We were staying in a hotel room in Greece and I had invited two gentlemen to come and fuck her in front of me. She didn't know of my plan and a few moments before they arrived, I asked her to dress nicely and then blindfolded her. The guys then arrived...
For some reason it didn't flow and we stopped the experience halfway. The guys were experienced swingers, so they were cool and understanding - they were actually the ones who came up with ideas to make sure my gf would feel safe and comfortable the whole time.
I guess my point is:
• if you don't already know the guy(s) joining you, make sure they're respectful people.
• it might be a good idea not to do it at your home, and keep your home as a emotionally safe place to retreat to in case something goes wrong.
• maybe agree on a safeword if your gf wants to interrupt the experience anytime.
I'd love to be the 'other guy' is this kind of fantasies, and I'm curious about the scenarios you came up with. Maybe I could help!