Some groups are not stretched, but if you are talking about a group of parts, you can do something like this:
<textField isStretchWithOverflow="true" isBlankWhenNull="true">
<reportElement style="base" positionType="Float"
isPrintRepeatedValues="false" x="0" y="3"
width="380" height="26" isRemoveLineWhenBlank="true"/>
<textElement/>
<textFieldExpression class="java.lang.String">
<![CDATA[$P{information}]]></textFieldExpression>
</textField>
This is pasted from automatically generated XML, so there are a lot of superfluous things, but isStretchWithOverflow = "true" should work for you. This will make the field stretched when the text fills it.
iReport , . XML.