I suggest you start out with something small and work your way up. My wife and I started with me watching her give a massage and hand job to a hung black guy. We wanted to test the waters for both of us. If it didn't work out, no harm and no foul. It was just a hand job, not enough to break us up or cause a big scene. It turned out that we both liked it and invited him back for a blow job. That went well too. On the fifth visit, they had sex. It was all great. No pressure, no nervousness, just excitement.
Don't go too far and she's likely to agree.
Even if you start with kissing and flirting, that's enough to test the waters. And you don't have to go all the way either. I know couples who enjoy having the wife dance close and sexy with another guy while the husband watches, that is enough for them and they are happy.
There are no rules but it's best to start small and take small steps to find your level and what you both are the most happy and comfortable with. Most of all -- HAVE FUN!