From man visudo:
visudo locks the sudoers file against multiple simultaneous edits, pro-
vides basic sanity checks, and checks for parse errors
There's nothing wrong with editing it with vi or any other text editor without visudo, as long as no one else edits it at the same time and you get the right syntax, essentially the same problems you have to aware of when editing any other config file.
Here's an example of how to give a user root access to run only a particular command using sudo:
alumno03 ALL=(ALL) NOPASSWD: /usr/sbin/groupadd
Note you can replace the NOPASSWD with ALL if you still want them to have to enter their password when they sudo.