sudo - How does the sudoers work?

During an installation, as usual, we create our main user account, and then we can do sudo commands with it without problem.
Now, when I created another account, and I wanted to do sudo, it gave me error that the account is not in the sudoers file. In that file I found out that users in %admin and %sudo groups can gain root privileges. But my main account is not in neither one of those groups and I can do sudo with it without problem, but with the other account I can't.
Is there something besides sudoers that I should know about? Thanks