simple add to admin group

[R.L. Reingard]

hi all
yesterday i fixed the 'sudoers disaster' simply by:

picking the "recovery mode"
AND putting my user back to the admin group:
$ adduser username admin

eventhough i like the fact, that i was able to fix the 'sudoers disaster'  
so quickly, i question myself now:

someone knowing the password of a simple user (one not in the  
sudoers-list) could start up the machine in 'recovery mode' and add that  
user by the same command to the admin group (the admin group, which has by  
default sudo rights).

is that nice?
what be glad if we can talk on that.
regards,
René