i think my best friend is bi?

Just let her know that you're there for her. Remember, she might not want to come out as she fears not being accepted. If you properly convey your views on the subject, and aren't judgemental or discriminatory, then she might feel more comfortable or willing. Until she comes out, just talk to her normally - don't make it seem like you know something she doesn't, as that could cause tension or mistrust within the friendship.

Treat her normally, and make sure you tell her that you'll be supportive regardless of anything she does, or is.

As already said, don't make assumptions as she might not even be bi: it could just be a rumour. And for the friend thing, it's normal for people to gain friends over time - that could be all it is. She could just have found people with common interests with her.

"been having long talks with people ive never seen her talk to before"- Maybe she has just met new friends? Or she already knew them but e.g. they aren't from the same school etc (I have friends from my athletics club that none of my friends know of) Maybe she isn't ready to have a conversation about it yet? you need to wait for her to pluck up the courage herself to tell you, not for you to have to ask. I don't see why she really needs to 'come out', only 1 of my friends in real life knows that I myself am bisexual (unless others I know are reading this comment, then hi, you also know) and I am totally cool right now with not having to tell people.
OR
she may not be bisexual at all and who ever told you was lying or heard false information themselves.