I realized that I'm bi, and I now have a girlfriend, but don't know how to come out to friends (NSFW)

Okay, so I’m 17, and recently, I’ve been really confused about some things… I have this friend, Melanie, and she’s 18. We’ve been friends for about a year and a half now. Melanie is bisexual, and I’ve always thought that I was straight, and although she’s always respected that I was straight, she’s always sort of harmlessly, playfully flirted with me, and I’ve flirted back, as it’s always been in a joking sort of tone. However, I have always thought that she was gorgeous. She’s much shorter than me; very petite with short hair and the most adorable smile. She’s always so happy, and she makes me feel better whenever I’m down. Well, in the past, I’ve only had two boyfriends, neither being very serious, and I’ve only been with one guy, sexually. Well, I’ve never had a serious attraction to another girl before, but recently, I realized that I’ve had this growing attraction to Melanie. Whenever she smiles, I melt, and she’s all that I think about… I’ve even fantasized about her when I get myself off. However, since I’ve never felt like this about a girl before, I’ve tried to deny it, and act normally around her.

The other night, I was at Melanie’s house, and her parents and her brother were out, so we were alone. We were in her room, changing into our bathing suits to go swimming. Once she had just finished changing, I took a deep breath and told her that I wanted to talk to her about something. She was really interested in what I had to say, and I told that I think I’m bi, and I asked her how she realized that she was bi. She told me that it was when she developed really strong feelings for this one girl, and asked me if I had feelings for any girl in particular. I told her yes, and she spent about 5 minutes trying to get me to tell her who before I admitted that it was her. She looked surprised, and then smiled at me, and pulled me towards her and kissed me. It was by far the best kiss I’ve ever gotten. She pulled me onto her, and we made out for a good twenty minutes before she started pulling my bikini top off. I was extremely nervous, since I’ve never been with a girl before, but she was really sweet and made me feel comfortable with everything. I followed her lead, and she started kissing my neck and ended up fingering me and going down on me. Afterwards, she asked me, “So you still think you’re bi?” and I told her, “Definitely.” So she smiled, and asked me “So then, if you’re definitely still bi, and you’re definitely attracted to me, would you want to go out with me?” I told her yes, but there’s a few problems.

Melanie and I have a similar group of friends. Most of my good friends have also become friends of hers, so we all spend a lot of time together as a group. Our group of friends has known that Melanie was bisexual since they met her. She has no problem telling people straight off the bat that she’s bi, and our friends are okay with it. However, most of these friends have known me much longer than Melanie; most of my life, and they’ve known me only as straight. I feel as though coming out to them would be difficult, as I’m afraid they would act differently around me; mostly my female friends, who are fine with LGBT people, but don’t exactly know how to act around them. I know that Melanie is really excited about us becoming an item; She told me that she’s “happier than she’s been in a really long time” because of our new relationship, and I feel the same. She told me that she would really like for our friends to know that we’re an item, because she’s so happy about it, and she “wants to be able to hold my hand or give me a kiss without hiding it from them.” She told me that she would never say anything or push me to come out to them if I’m not ready, though.
However, I don’t want Melanie to feel like I’m ashamed of my feelings for her and/or my relationship with her, because I know she’s excited about it. I do want our friends to know, but I’m not sure how to go about doing it. Should I put this off longer? I don’t want Melanie to feel hurt, or think that I want to hide the fact that we’re going out. How should I deal with this? What should I tell her and/or our friends?