If you are not able to change user group of particular user and any problem occurred then you can modify user group of user from SQL back end through update query using query editor.
Select * from USERGROUPINFO
Select * from USERGROUPLIST
To modify user group you can use following update statement to add user testuser to admin group of Axapta.
Update USERGROUPLIST set GROUPID='Admin' where USERID='testuser'