I suggest you do a tag search and/or advanced search here or the term "unicorn" and "unicorn hunter." You can find those functions by clicking on the Search link above.
Now, think about it - you don't really "add" a person to your existing relationship. You meet someone you like, and
relate to them, see if there's compatibility, then see if you both want to spend more time together, and so on, just like any other relationship starts out. People do not get added to whatever kind of partnership you have already - because human beings are not condiments for your hamburger and should never be treated like some
thing to add to or enhance the relationship you currently are in. Please read these threads:
"Added To, Joining In"
Triads, Unicorns, Unicorn Hunters, sexual 3somes, etc.
What is the appeal of a unicorn?
What's in it for a unicorn?
...and this article:
So, someone called you a Unicorn Hunter?
...and this book:
Opening Up by Tristan Taormino
You can find other books in our book recommendation thread - do a search.
Furthermore, just a note of caution (and I have a hard time understanding why this still has to be said in the year 2015) - if you are a mature adult, wouldn't you rather be in a relationship with a woman and not a girl? Do you call your husband a boy? Calling a woman by the same word that describes pre-pubescent females ("girl) is demeaning, a bit creepy, and demonstrates an inability to distinguish between age groups when relating to females. If you want to be in relationships with adults, and for everyone to relate to each other with maturity, capability, and consideration, I recommend you start thinking of and referring to yourself and others as adults.
If you do a lot of reading, asking questions here, as well as examining and discussing your motives for wanting a "girl" to "add to" your relationship, for a good long time
before you embark on practicing poly in your life, you might have a chance without causing too much damage. Some couples take a year or two of talking and deep introspection before taking the first step. Best of luck to you and your husband.