You need to add classes or package from which the groups would be picked up. Just the groups tag won't work.
Adding an example from testng documentation (you can specify a package as well based on need):
<test name="Test1">
<groups>
<run>
<include name="functest"/>
</run>
</groups>
<classes>
<class name="example1.Test1"/>
</classes>
</test>