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'