Need to verify, is this really a “group” feature (method of navigation), or direct scroll control with custom key codes. Those things are completely different.
Right now i remember only one “missed” navigation feature - “subgroup”. When user may enter subset of elements and leave back to upper level. Web has nothing similar. This can be avoided by alternate design of navigation - instead of subgroup, show new popup page with elements for that “subgroup”.
I’m not very experienced about web accessibility features. But if you will have concrete questions or cases to investigate - kick me, i’ll try to reread specs.