You are being jealous; but I that's pretty natural. Your friends are diverting their attention away to a shiny new thing. It's a pretty natural response.
Something I want to address right away is if you are upset about not being the 'leader' any more (social meritocracy worry me anyway >_>) or if you feel like you are being excluded from the group.
If you are upset about not being leader you are just gonna have to deal with it; either one of two things are going to happen, this girls shiny new novelty appeal will wear off and things will go back to how they where; or she is more charismatic and interesting than you (you said she is more outgoing yourself) and honestly you can't change that without some sort of nasty social engineering attack to discredit her.
If you feel like you are being excluded then that's a bit of a different bunny. If you are in a mostly female group it would most likely be best to talk it out as
>>4 suggests. If your in a mostly male group then that might be counter productive and it might be best to talk to the girl in particular.