You can provde the dashlet family name in the desc.xml file of the dashlet.
For example : In th following file, the family name is "dashlet" so you can see it in user dashboard.
tomcat\webapps\share\WEB-INF\classes\alfresco\site-webscripts\org\alfresco\components\dashlets\site-search.get.desc.xml
If the family is "site-dashlet", you can only see it under site in alfresco share.
You can provide <family>dashlet</family> in your desc.xml file.