I've just read about How To Fix Broken sudo Command in Ubuntu and I used grub menu on my linux virtual machine to solve my problem and add my user account to sudo group. So I would like to know if in large system of a company, can a user do the same thing as me and be able to use sudo. My guess is that they aren't in direct interaction with the machine like in my case, but I just want to clear my doubts. Thanks in advance.
If you can gain access to a system's console there almost no limit to what you can do to the system if the system does not have full disk encryption (and even then there are other things to be concerned about, bios for example).
Situations where someone can access the console (like accessing the grub boot loader) or access the hardware requires different approaches to secure a system from malicious access (look into the evil maid scenario).
If you protect physical access to the system, then on most well secured systems, escalation of privileges is non-trivial.