To search for ungrouped Users, on the Left Navigation click "Users". Next to the search box, click "Advanced Filters" link.
In the Advanced Filters popup window, select “Ungrouped” from the drop down menu next to the Groups section. Next, click Search.
Tip: You can add additional values as part of this search to find exactly the Users you need.
Now, You will see a list of Users who are not a part of any Group.