I think too much emphasis gets put on terminology. You can have a 20 year committed pentad, without ever using the words "polyamory" "group marriage" or even "commitment." Titles are superfluous. What matters is that everyone knows how everyone else feels, and encourages each other to have whatever feelings come naturally.
The advantage of "just letting it happen" is that you avoid a lot of bloated expectations. People have so many ideas of what "a relationship" should be like, that they forget to just be people enjoying one another.
Eventually, you get to a point where it's pretty obvious what's going on. It sounds like you're there. So if it is time to talk about it, don't get bogged down in labels. Just tell her that you like her as "more than a friend" and you'd like to explore those feelings with no expectations or reservations.
One thing I feel the need to point out: From what you've said, it sounds like she may not be interested in your husband romantically. She holds your hand and tells you she loves you. Have you and your husband discussed how he would feel if she wanted to be romantically involved with you but not him, even if he's also developing feelings for her? Will he be willing to do the hard work of dealing with envy and jealousy while you explore a romantic relationship with her? Will he be willing to put his desires in a locker, so that she doesn't feel uncomfortable, that her best-friendship is being eroded by his unrequited love? Just some things to think about...
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."