You do both need to be 100% sure about everything going into this.
My wife and I both have others we meet, often alone, and occasionally as a threesome with her main FwB, but we only do this after we had a series of very honest and frank discussions about it.
Will it just be one person who you have regular sex with.
How often would this be? A weekly or monthly limit?
What about overnights? Would this be at your place or his.
Would staying in a hotel be acceptable, if so just one night or a weekend or longer away.
What if you meet someone else you would like to have sex with.
What is out of bounds? For us, it's anal sex.
Would you kiss each other? A greeting kiss or passionate full-on kissing, which we do with all our partners.
Would you inform your husband of what you had done or would that remain private. We find it is great discussing what we have been doing
Would you be OK if he decided to have sex with other women without you?
We both get great joy from having sex with others and there has never been a single pang of jealousy, in fact it enhances our own sex life.
Good luck in whatever you both decide together.