Which version of Orion are you using?
It works for me when I use curl and post with the data below
I'm running the latest greatest orion 10.4.2. What I didn't notice was that the group was being created (by the time I did, I had created 4 groups named the same).
It is spitting out the the same error message 'Cannot serialize parameter of type 'System.Int32'...' where it should be returning the ContainerID instead.
I was able to reproduce this problem on some machines but not others. I'm still investigating, but I found a possible workaround for you: installing .NET 4.5 on the Orion server seems to make it work. I hope to find a fix that does not require .NET 4.5, but in the mean time you could try that unless you are using Window Server 2003.