curious..what kind of things would you be whispering to her?
It's not so much what you say, it's that you're saying it only to her. It shows her that you only care about her and she should only care about you and ignore the people she's with. It's like the two of you are alone even though there's people with you.
She said out loud to me "I think we're going to go to [some restaurant I think]. I whispered to her "You're mine. You're here with me now. Forget about them. It only matters what you want." and escalated furthur.
Weak sauce. You knew that you should have asserted your dominance over that group. You should have likewise asserted your dominance over her, as she was clearly a follower, looking to the comfortable familiarity and security of others for her cues- as she aptly demonstrated when she went along with her bossy friend's lame restaurant plan- even though she really wanted to be with you. Therein was your clue to her submissive character- and your fatal mistake.
And bullshit. You most certainly wanted to "influence her decision"- in fact, you were attempting to control the outcome all along. To make it appear as though you were not influencing the decision was not the appropriate tactic in this case. She wanted you to take charge.
There is a retricence in you that does not suit your potential. You have the skills, but perhaps you lack a measure of confidence. Am I right to assume you're quite young?