I am often confronted with the same task. I've added a bunch of students to a course page by importing their names from a backup of another course (on a different Moodle) . I then want to add all of those students to a newly created group. Currently I have to find each of their names from among ALL students currently enrolled in the course, one by one.
I suggest including a button at the top of the screen (in group/members.php) that says "Add all students who donot belong to any group to this group).