Actually, in this case, it is a bug. Even though exclusion groups are "fields" per se, they still have some of the same attributes and they're the ones that store the value of the selected radio button within their group. Therefore, one should be able to set an exclusion group (i.e. radio button group) as "required".
Unfortunately, there's a bug in Acrobat which ignores the "required" attribute of exclusion groups upon data submission. This bug has been reported and will hopefully be fixed in a future release.
For the time being, you can use Danmaster's solution or you can use another solution which uses two buttons: The first is the actual submit button (either email or HTTP) and is invisible. Let's name it "EmailSubmitButton". The second is a regular, visible button which has script in its Click event which will verify that a value has been specified for the exclusion group. If a value has been specified, the script then executes the invisible submit button's Click event which will then trigger the form's data to be submitted. Otherwise, the script can show an error message.
The following script is an example of what you would put on the regular button's Click event (in JavaScript):
if (RadioButtonGroup.rawValue != null) // user picked a radio button
EmailSubmitButton.execEvent("click"); // submit data
else
app.alert("Please pick an option in the list.");
Note that this technique is also very useful for validating the form prior to submitting or printing it if you have to do more complex validations aside from setting some fields as "required".
Stefan
Adobe Systems
