Friend had sex while I was in the room 😷😱

So i went round to my friends house for a small party, she'd said there were only 7 people there. I got the at 11pm because I went out with another friend before. When I got their on knocked on the door, her mum answered and said their not back yet( her mum was going out later). So I went and sat in for room, texting and calling her, because I was on time and she knew I was coming. After about 45 minutes she comes in, with another friend from school ( let's call her Megan ) and two boys. When she saw me she said she was sorry and that they'd gone to get Megan. The 7 people people she'd said would be there weren't and I was already annoyed. The boys were about 17, we are 14. 
We sat in her room, listening to music and drinking a little. As I'm talking to Megan. My friend saddles this boy and they were proper snogging, we all just try and ignore it. We were all sat on her bed and they kept falling back on us, not stopping until we yelled at them to sit back up. Later they go onto the floor and kissing and rolling around, breathing real heavy. Later me and Megan try to get to sleep. And the other three are on the floor because she hasn't got us all beds! We then swap with my friend and the boy so they can get some sleep. But no they start breathing really heavy and the beds like rocking, it went on for hours. They were trying to be secret by saying they'd got lots of sleep but it's disgusting. Bearing in mind nearlier she'd taken her jeans off because she was to hot, so she was just wearing a top and a thong!! And I later found out, she'd only known the boy two days!!!! 
She has no respect for herself and I feel really disrespected. 
She's invited me to another 'party' soon with afew more people I know, but I don't know whether to go ( the boy and his friend are both going aswell)Â