How to log in as an administrator and return his personal ID on the user management page. Using "get_current_user_id()" will display the administrator's ID instead of the user's ID.
You can create a text field (top field, not a subfield in a group) for the user with the field id user_id > enable admin column for this field > edit a user > just click on Update user.
Then you will see the user ID field value displayed in the admin column (user management page).