Have you ran the upgrade script? It sounds to me like you have not ran the upgrade. The usergroup captcha field is added in 8.1 and if you did not run your upgrade meaning that field does not exist that means we can not grab the usergroup info. You will never be an admin till you run the upgrade for 8.03 to 8.1 to add in the required fields.
The upgrade instructions are very clear you upload everything run the upgrade and then for vb4 you can go to adm-index.php which is admin and redo the xml provided you have applied all proper permissions.
Number one issue with upgrading is no one reads the documentation.