Node groups are really buggy right now. Aside from the lacking compatibility between the Cycles inputs and the Octane nodes, there are also issues when using Octane inputs only. As you can see in the attached video, the inputs aren't transported to the nodes correctly. You can also see that when you manipulate the values outside the group, strange flickering happens with the material rendering.
I think that's something that should looked into rather soon. Node groups are a huge part in Blender and everybody uses them regularly.
Octane 28.9 CB, Windows 10, RTX 3060