The easiest way to approach her is probably with friendship first and foremost in your mind, with the possibility of something more tucked neatly in the back.
I'm encouraged by you saying "one or both of us." It's important to realize that one person very rarely falls for two members of a couple equally, if at all. Many couples, when they first learn about polyamory, have unrealistic expectations of "adding" another person to their relationship. People are not accessories. But it doesn't sound like you have that problem, so that's encouraging.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker