It’s completely your choice whether to indulge yourself in sexual activities or not and with whom. You are not answerable to anyone for this. Here, your girlfriend and her friends have no right to harass you. In my opinion, you should probably need to rethink about your relationship with your girlfriend as she, herself is asking you to indulge in sexual activity with other girls. Have a proper conversation with your girlfriend about this and confront her about why she did that because this is not a small matter for you to ignore.