Register a new Efl.Ui.Radio button to this group. Keep in mind that registering to a group will only handle button grouping, you still need to add the button to a layout for it to be rendered.
If the Efl.Ui.Radio.state_value of the new button is already used by a previous button in the group, the button will not be added.
See also Efl.Ui.Radio_Group.unregister.
Since 1.23
register @pure_virtual { params { @in radio: Efl.Ui.Radio; } }
void efl_ui_radio_group_register(Eo *obj, Efl_Ui_Radio *radio);