Good evening all.
I had the same problem after upgrading the site from J 1.5.26 to 3.2.1 with the SP Upgrade component with witch I imported all needed tables.
In 1.5.26, the "user group" is in the x_users table under the field "usertype"
In 3.2.1 you find a new table "x_user_usergroup_map" in witch you find the link between "user_id" and "group_id". SP Upgrade creates records in the new table "x_user_usergroup_map" (one per user) and gives as value "0"(no group) in "group_id" for all users excepted for the superuser.
Replacing with PHPAdmin/sql "0" with "2"(registered) for all users solved the problem of displaying the users in the Back-end / CB User Management
I suppose I will probably face some other similar problems with user lists...