I think this may be a bug or incorrectly documented at https://docs.cs-cart.com/4.11.x/user_guide/users/administrators/admin_group.html?highlight=user%20groups :
If the Customers > Administrators > Admin account has the Firstname / Lastname fields changed to anything except 'Admin' then the User Groups tab is not visible while trying to allocate other administrators to different administrator groups.
I have confirmed this on my own site by checking the MySQL table
via unix root user:/# mysql -u database_username database_name -p database_user_password
MariaDB [database_name]> select * from cscart_users
\q
user_id: 1 user_login : admin is_root : Y firstname : Admin lastname : Admin