Regina is right - the grouping is done with "form:name" attribute, already way back in ODF 1.1 (although it's quite a bit more obscure in the old version)

"11.5.13 Selected
The form:selected attribute specifies the default state of a radio button or option. When the control is initialized, it is in the default state specified by this attribute. This attribute is supported for the following elements:
<form:option>
<form:radio>
In a group of radio buttons that share the same name, only one radio button can have this attribute set to true."

There is no need for such an element. Grouping of radio form elements is done by the attribute form:name.
Read in section 13.5.18<form:radio>
"Radio buttons are defined to belong to the same group if they have the same control name, as specified by their form:name attribute.
If a group of radio buttons is bound to one database field, and a user selects any given button, the reference value of the selected radio button is written into its database field."

That is the same behavior as known from HTML.