Documentation link which you have give is correct.It should work in your case.Its better if you can include code.
I think you might be misundedrstood below thing.In alfresco group name has a prefix which is "GROUP_"
If you want to give access to specific group for particular dashlet you can give it.For example if you would like to give access to dashlet to particular group named as Workflow_Users, than family tag should be like below.
I thoughtit's my misunderstanding of the tutorial. What I expect is the final dashlet will restrict only for particular users of specific group. But after read it again, the "group" parameter is only restrict availability of dashlets list in Customize Site/User Dashboard page. Hence if user is not member of particular group defined in descriptor file, he/she will not see the dashlet (in Customize Site/User Dashboard page).
My first question is still unanswered: How to restrict dashlet appearances for a particular group?