You can get IBuildDetail object for build from context extensions (or pass it to activity as argument)
context.GetExtension<IBuildDetail>()
Properties from there will provide what you need
http://msdn.microsoft.com/en-us/library/microsoft.teamfoundation.build.client.ibuilddetail.aspx
Public property RequestedBy Gets the user who requested this build.
Public property RequestedFor Gets the user for whom this build was requested.