The community I am in has had sex parties where my friends got together and had group sex. Here is a link to a thread on it http://www.polyamory.com/forum/showthread.php?t=1995
I was completely overwhelmed by my feelings on it. I felt left out and astounded that they would want to do such a thing and that they wanted me to join them. Instead of love and acceptance I felt horror. I realize now that it was because of how I connect when I am sexual with someone. Also my past experiences catching up with me.
The thought of connecting with all of them on that level was overwhelming and unappealing. As well as just not possible. I would have to shut off myself in order to achieve the act of sex, as I had when I was a swinger for a short time. It isn't my idea of a good time unless its with people I want to connect to in that way. I relied on my community to be just that, community. It was akin to fucking the guy who bagged my groceries last night and I had a nice conversation with. Ya he was interesting and we bonded over the windy weather, but that was it... no other attraction other than a warm feeling of commonality.
I was afraid that my response to the event would mean losing friends and I was devastated as I had invested in friendships and the new community that I was growing to love as a whole. As it turned out there was a bit of backlash for some, a bit of sealed relationships that happened and a whole lot of moving on. I worked through my issues and got right back on the horse with my community before letting my lack of self esteem overwhelm me so much that I would lose the investment I had. To my joy people met me half way and accepted my craziness enough to patiently watch as I went through shit...
now it seems we are all good. If they were to have an event like that again I would wince because of my own thoughts, but at least would understand who I am within that and could move on much more easily.
Apparently though, to answer you question. It is actually very common in some communities to have group sex. There is a community that I know of that make it a monthly event! So.... you are not alone it seems.