Try using the path to the FeatureServer itself. You should get a parent composite layer ("group") that contains all of the individual layer end points (however many there are).
LayerFactory.Instance.CreateLayer(new Uri("http://....url here.../FeatureServer", UriKind.Absolute), MapView.Active.Map);
Thanks for the response, Charles. Adding the feature server does create a group layer containing all the feature layers.
A follow-up question: it is possible for the FeatureServer to get updated with a new layer. How would I add this new layer to the group, or somehow "refresh" the group layer for any new layers? I tried something like:
// groupLayer = the existing group layer
// "layer 5" is the newly available layer in the FeatureServer
LayerFactory.Instance.CreateLayer(new Uri("http://... url here.../FeatureServer/5"), groupLayer);
But returns null as before.