My friend has recently come out as bisexual, how do I support her?

To cut a long story short, for a while now my friend has been seeing someone. Until very recently she had kept us hanging as to who this person was until wednesday when she let us know by group chat that she was seeing another woman. We are a very close group of four friends and I feel quite embarrassed that she didn't feel comfortable enough to tell us in person.

I am bisexual myself, and I knew exactly how she felt, but when it came to telling her that, I had no idea how to say it!! I just froze, I think it was partly the shock of finding out she was also bisexual (because I never would have guessed!) and partly because her awkwardness around the subject made me revert right back to my 16 year old self and relive all the anxiety I felt when I first realised I liked women.

This particular group of friends have only ever known me to be with men so did not know I was bisexual. I want to be able to tell her I know exactly how she is feeling, but I just don't know how to talk to her about it and have a proper honest conversation.