Is your daughter sexually active? How else would she understand the meaning of bisexual? Sounds like you've been discussing being bisexual and asking her if she is. "She admits liking both boys and girls." Why does this upset you? Why do you think liking boys and girls means she's bisexual?
I suggest that because you're focused on her sexuality she is thinking that she has to answer your questions. She's 11. Who she has sex with should not be an important part of her life. School, making and keeping friends, exploring activities. Is she on a sports team, learning music, be a part of a group focused on activities?
Of course she's wondering about sex. She has questions. Unless she's sexually active how would she know if she's bisexual? I suggest she and the friends you mention are trying to understand sexual feelings by imagining what being bisexual is like. I suggest kids "try out" roles. It's common for girls to kiss each other as they explore their sexuality. At 11, girls are trying out various situations; not just those related to sexuality. They "decide" on a career. They talk about who they want to be when they grow up.
I suggest that you talk with a counselor to learn about girls this age and what to expect from your tween daughter. Talk about why you think she's going to be bisexual her whole life. Ask for ideas to help you to respond to your situation. If that counselor asks for your daughter to be included, include her.
Parents join support groups because parenting is a tough job.
BTW I like men and women. Once in awhile I am sexually attracted to a specific woman. I don't act on that feeling. I'm definitely straight. Often, I feel sexually attracted to men. I rarely act on those feelings. We are sexual people.
A later comment. Of course she's trying to fit in. That's what we do, even as
adults. When we don't feel a part of a group we move on. As your daughter matures she will change what fitting in means to her. Now she is 11 and learning about friendships. Her friends
are girls her own or close to her own age. They're all immature and learning about friendships. Your daughter will figure out about life and how she wants to relate to others. It's normal for her to agree with friends. Saying they're bisexual now does not mean they are or even that they'll say it in 6 months.