Sounds to me like you're moving too fast, probably talking about relationships, scaring her a little.Just slow things down and wait for her to bring up relationships. When a woman is ready, she'll bring it up. Don't go telling her how you really like her and bla bla bla. Remain mysterious, let her wonder about you, questioning where she stands with you etc.
It's a scientific fact that women are more attracted to men whose feelings are unclear. So you're going to need to make her think that she likes you more than you like her. (Don't be a dick, just stop putting her on a pedestal)
Your attitude, and response, should be something along the lines of:
Dating somebody else, huh? That's ok, I'm spending time with other girls too. I don't want to get locked down to any kind of commitment just yet. Just take it slow, hang out, have fun, and don't worry about relationships or whatever else for now. Ok?
1. You're setting the tone
2. She probably said the same thing to the other guy - it's just a test, strongest reply wins. Don't get all broken over a minor obstacle like that.
3. She'll think she was being silly
4. She'll get a little jealous
However, make sure you remain mysterious about "other girls" -
don't elaborate on this. She'll probably either pretend to be mad, or throw some shit test at you. But the bottom line is,
women like you more when you've got options.
However she responds, just act calm and play it cool. It'll work magic in the long run.
You want to invite her to something with friends? I wouldn't. No group dates until shes in love with you (which, if you're doing everything right, should take around 2 months, according to statistics.) Until then you should just be phoning her
once a week to arrange a definite date (don't leave anything in the open, arrange a time and place to meet her out, or pick her up.) Arrange it for 3-4 days into the future so that she'll be anticipating it and letting the mystery build in her head. Don't even tell her where the date will be. Just give her something like "wear old clothes - you'll get dirty" or "dress to impress

" Check the new article on
http://www.ipickupwomen.com for some first date ideas.
The major thing here is: If you want to be her friend, do things that her friend would do (group activities like pool party's) but if you want to be her lover, do things that her lover would do (1 on 1 dates, flirting, escalating things into the bedroom.)
Don't worry about how you can make her your girlfriend, or locking her down to any kind of commitment.
"The need for anything, gets in the way of everything" think about that.
Just contact her once a week, date other women, keep your options open, and see yourself as the prize. Stop thinking that you have to win her over. How do you know shes good enough for you? Like she said, you've only met a couple of times. Just have some fun.
Hope I helped.