Sounds like you approached it perfectly! And nice and slow is the perfect pace to start.
One thought immediately springs to mind. Often when there's a couple and a third person, attraction may start out equal then become more lop-sided. She may end up falling for you but not your husband, or falling for your husband but not you. Would those potential scenarios be ok with him? With you?
This essay may provide some useful thoughts as you move forward: http://www.morethantwo.com/coupledating.html
Best of luck!
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.