I've been the +1 male.
I wasn't particularly looking, I was on a dating site and was approached. It transpires I was recognised from an old workplace, so they felt a bit safer half 'knowing' me.
We had a lunch meet at a pub, to discuss things and make sure there was a spark. The initial ask was I would simply watch them, something I agreed to. We returned to their place.
As you can imagine with a naked couple in front of me, things took a turn and I was invited by her mainly to join in.
It was good to have chatted things through, it also meant during the event it was easy to check in and be respectful of them. I'm not bi, neither was he, but you definitely need to be comfortable close and contacting each other, in fact it was part of the excitement for me, and ended up with some interesting helping hand moments...
Do think through practicalities - we had a very long afternoon of things. Multiple condoms needed. A warm room. Somewhere to clean up.
Enjoy. It's one of the most enjoyable things I've done sexually, I'd happily do it again.