I'm guessing you mean "grouper" instead of "group", as I don't recall there being a "group" property.
In build 286 we have enhanced the configuration of groupers so that a Grouper is a separate toolbox item, which you can add by dragging it onto a Store. It can then have its properties edited using the config editor for convenience. If you want to supply a custom function like you used to, simply put it in the groupFn config on the Grouper instance.
Sencha Architect Development Team